Document and Entity Information - shares	3 Months Ended
Document and Entity Information - shares	Mar. 31, 2016	Apr. 30, 2016
Document And Entity Information [Abstract]
Document Type	10-Q
Amendment Flag	false
Document Period End Date	Mar. 31, 2016
Document Fiscal Year Focus	2016
Document Fiscal Period Focus	Q1
Trading Symbol	AKBA
Entity Registrant Name	Akebia Therapeutics, Inc.
Entity Central Index Key	0001517022
Current Fiscal Year End Date	--12-31
Entity Filer Category	Accelerated Filer
Entity Common Stock, Shares Outstanding		37,976,705

Condensed Consolidated Balance Sheets - USD ($) $ in Thousands	Mar. 31, 2016	Dec. 31, 2015
Current assets:
Cash and cash equivalents	$ 43,439	$ 49,778
Available for sale securities	173,586	88,676
Prepaid expenses and other current assets	2,505	2,563
Total current assets	219,530	141,017
Property and equipment, net	629	540
Deferred offering costs		102
Other assets	1,281	1,281
Total assets	221,440	142,940
Current liabilities:
Accounts payable	2,415	2,313
Accrued expenses	10,994	9,555
Total current liabilities	13,409	11,868
Deferred rent, net of current portion	257	69
Deferred revenue, net of current portion	40,000
Other non-current liabilities	12	5
Total liabilities	$ 53,678	$ 11,942
Commitments and contingencies (Note 8)
Stockholders' equity:
Preferred stock $0.00001 par value, 25,000,000 shares authorized at March 31, 2016 and December 31, 2015; 0 shares issued and outstanding at March 31, 2016 and December 31, 2015
Common stock: $0.00001 par value; 175,000,000 shares authorized at March 31, 2016 and December 31, 2015; 37,946,010 and 30,662,218 shares issued and outstanding at March 31, 2016 and December 31, 2015, respectively	$ 0	$ 0
Additional paid-in capital	355,138	292,783
Treasury stock, at cost, 8,463 shares	(162)	(162)
Accumulated other comprehensive loss	(27)	(234)
Accumulated deficit	(187,187)	(161,389)
Total stockholders' equity	167,762	130,998
Total liabilities and stockholders' equity	$ 221,440	$ 142,940

Condensed Consolidated Balance Sheets (Parenthetical) - $ / shares	Mar. 31, 2016	Dec. 31, 2015
Statement Of Financial Position [Abstract]
Preferred stock, par value	$ 0.00001	$ 0.00001
Preferred stock, shares authorized	25,000,000	25,000,000
Preferred stock, shares issued	0	0
Preferred stock, shares outstanding	0	0
Common stock, par value	$ 0.00001	$ 0.00001
Common stock, shares authorized	175,000,000	175,000,000
Common stock, shares issued	37,946,010	30,662,218
Common stock, shares outstanding	37,946,010	30,662,218
Treasury Stock, shares	8,463	8,463

Condensed Consolidated Statements of Operations and Comprehensive Loss - USD ($) $ in Thousands	3 Months Ended
	Mar. 31, 2016	Mar. 31, 2015
Operating expenses:
Research and development	$ 20,235	$ 6,664
General and administrative	5,811	4,232
Total operating expenses	26,046	10,896
Operating loss	(26,046)	(10,896)
Other income (expense):
Interest income (expense), net	234	96
Other income	14	105
Net loss	$ (25,798)	$ (10,695)
Net loss per share applicable to common stockholders-basic and diluted	$ (0.70)	$ (0.53)
Weighted-average number of common shares used in net loss per share applicable to common stockholders-basic and diluted	36,873,594	20,030,129
Comprehensive loss:
Net loss	$ (25,798)	$ (10,695)
Other comprehensive loss - unrealized loss on securities	(26)	(8)
Comprehensive loss	$ (25,824)	$ (10,703)

Condensed Consolidated Statements of Cash Flows - USD ($) $ in Thousands	3 Months Ended
	Mar. 31, 2016	Mar. 31, 2015
Operating activities:
Net loss	$ (25,798)	$ (10,695)
Adjustments to reconcile net loss to net cash used in operating activities:
Depreciation expense	32	19
Amortization of premium/discount on investments	144	135
Stock-based compensation expense	1,251	1,128
Changes in operating assets and liabilities:
Prepaid expenses and other current assets	58	(801)
Accounts payable	102	2,308
Accrued expenses	1,457	(411)
Deferred revenue	40,000
Other liabilities	188	(30)
Net cash provided by (used in) operating activities	17,434	(8,347)
Investing activities:
Purchase of equipment	(109)	(200)
Proceeds from maturities of available for sale securities	15,314	9,585
Purchases of available for sale securities	(100,160)	(5,980)
Net cash (used in) provided by investing activities	(84,955)	3,405
Financing activities:
Proceeds from the issuance of common stock, net of issuance costs	61,166
Proceeds from sale of stock under employee stock purchase plan		50
Proceeds from the exercise of stock options	19
Payments on capital lease obligations	(3)	(1)
Net cash provided by financing activities	61,182	49
Decrease in cash and cash equivalents	(6,339)	(4,893)
Cash and cash equivalents at beginning of period	49,778	32,780
Cash and cash equivalents at end of period	43,439	27,887
Non-cash financing activities:
Unpaid follow-on offering costs	$ 20	55
Assets acquired under capital lease		$ 12

Nature of Organization and Operations	3 Months Ended
Nature of Organization and Operations	Mar. 31, 2016
Accounting Policies [Abstract]
Nature of Organization and Operations	1. Nature of Organization and Operations Incorporated in Delaware in 2007, Akebia Therapeutics, Inc. (Akebia, or the Company) is a biopharmaceutical company focused on the development of novel proprietary therapeutics based on hypoxia inducible factor, or HIF, biology and the commercialization of these products for patients with serious unmet medical needs. HIF is the primary regulator of the production of red blood cells, or RBCs, in the body and a potentially novel mechanism for the treatment of anemia secondary to chronic kidney disease, or CKD. Pharmacologic modulation of the HIF pathway may also have broader therapeutic applications in acute renal failure, organ protection, ischemia-reperfusion injury, cancer, ophthalmology, and inflammatory diseases. The Company’s lead product candidate, vadadustat, formerly known as AKB-6548, is being developed as a once-daily, oral therapy for the anemia of chronic kidney disease. We have successfully completed Phase 2 development demonstrating that vadadustat can safely and predictably raise hemoglobin levels in patients with anemia related to CKD. The Company’s operations to date have been limited to organizing and staffing the Company, business planning, raising capital, acquiring and developing its technology, identifying potential product candidates and undertaking preclinical and clinical studies. The Company has not generated any product revenue to date, nor is there any assurance of any future product revenue. The Company’s product candidates are subject to long development cycles and there is no assurance the Company will be able to successfully develop, obtain regulatory approval for or market its product candidates. The Company is subject to a number of risks including, but not limited to, the need to obtain adequate additional funding including the resources necessary to fund its recently commenced global Phase 3 development of vadadustat, in dialysis and non-dialysis patients. In December 2015, the Company began dosing patients in its Phase 3 vadadustat program in non-dialysis patients with anemia related to CKD, PRO₂TECT, after obtaining feedback from United States and European regulatory authorities regarding the design of the program. The Company also expects to initiate its Phase 3 vadadustat program in dialysis-dependent CKD patients, INNO₂VATE, in 2016, anticipating full enrollment by early 2018. The Company has engaged a clinical research organization for the PRO₂TECT and INNO₂VATE programs. The Company expects the cost of the Phase 3 program to be in the range of $80,000 to $85,000 per patient and it plans to enroll approximately 3,100 patients in PRO₂TECT and approximately 2,600 patients in INNO₂VATE. The Company is also subject to a number of risks including the need to obtain adequate additional funding, possible failure of preclinical testing or clinical trials, the need to obtain marketing approval for its product candidates, the development of new technological innovations by competitors, the need to successfully commercialize and gain market acceptance of any of the Company’s products that are approved and the ability to protect its proprietary technology. If the Company does not successfully commercialize any of its products, it will be unable to generate product revenue or achieve profitability. Through 2015 we have raised approximately $187.4 million of net proceeds from three underwritten public offerings, including our Initial Public Offering, or IPO. In January 2016, we completed a follow-on public offering whereby we sold 7,250,000 shares of common stock at a price of $9.00 per share. The aggregate net proceeds received by us from the offering were approximately $61.0 million, net of underwriting discounts and commissions and estimated offering expenses payable by us. In December 2015, we entered into a collaboration agreement with Mitsubishi Tanabe to develop and commercialize vadadustat in Japan and certain other countries in Asia for total milestone payments of up to $350.0 million, including up to $100.0 million in upfront and development payments, of which $40.0 million was received in January 2016. If Japanese patients are not included in either the global Phase 3 PRO₂TECT or INNO₂VATE programs, $20.0 million of the $40.0 million received would be used to fund further local development of vadadustat in Japan or be refunded to Mitsubishi. In addition, we will receive tiered double-digit royalty payments on sales of vadadustat. The Company believes that it can continue as a going concern as the Company ended the first quarter of 2016 with cash, cash equivalents and available for sale securities of $217.0 million and expects its cash resources to fund its current operating plan through at least the second quarter of 2017. There can be no assurance, however, that the current operating plan will be achieved in the timeframe anticipated by the Company, or that its cash resources will fund the Company’s operating plan for the period anticipated by the Company or that additional funding will be available on terms acceptable to the Company, or at all.

Summary of Significant Accounting Policies

3 Months Ended

Mar. 31, 2016

Accounting Policies [Abstract]

Summary of Significant Accounting Policies

2. Summary of Significant Accounting Policies

Basis of Presentation

The accompanying condensed consolidated financial statements include the accounts of the Company and its wholly-owned subsidiaries, Akebia Therapeutics Securities Corporation and Akebia Europe Limited. All intercompany balances and transactions have been eliminated in consolidation. These condensed consolidated financial statements have been prepared in conformity with accounting principles generally accepted in the United States of America (U.S. GAAP. Any reference in these notes to applicable guidance is meant to refer to the authoritative United States generally accepted accounting principles as found in the Accounting Standards Codification (ASC) and Accounting Standards Update (ASU) of the Financial Accounting Standards Board (FASB).

In the quarter ended December 31, 2015, the Company identified and corrected an error in the historical classification of certain operating costs between research and development and general and administrative expenses. The Company concluded the effect of this classification error was not material to its consolidated financial statements for any prior period. The classification correction had no effect on the Company’s current or historical total operating expenses or net loss.

The results of operations for the interim periods are not necessarily indicative of the results of operations to be expected for the full year. These interim condensed consolidated financial statements should be read in conjunction with the audited consolidated financial statements as of and for the year ended December 31, 2015, and the notes thereto, which are included in the Company’s Annual Report on Form 10-K (File No. 001-36352), which was filed with the Securities and Exchange Commission (SEC) on March 14, 2015.

Recent Accounting Pronouncements

From time to time, new accounting pronouncements are issued by the FASB or other standard-setting bodies and adopted by the Company as of the specified effective date. Unless otherwise discussed, the Company believes the impact of recently issued standards that are not yet effective will not have a material impact on its financial position or results of operations upon adoption.

In March 2016, the FASB issued ASU 2016-09, Improvements to Employee Share-Based Payment Accounting, as part of its Simplification Initiative. The areas for simplification in this update involve several aspects of the accounting for share-based payment transactions, including the income tax consequences, classification of awards as either equity or liabilities, and classification on the statement of cash flows. Some of the areas for simplification apply only to nonpublic entities. The amendments in ASU 2016-09 are effective for fiscal years beginning after December 15, 2016, and interim periods within those fiscal years. Early application is permitted for all entities. The Company is currently evaluating the impact of this new standard on its consolidated financial statements.

In February 2016, the FASB issued ASU 2016-02, Leases (Topic 842), which supersedes the existing guidance for lease accounting, Leases (Topic 840). ASU 2016-02 requires lessees to recognize leases on their balance sheets, and leaves lessor accounting largely unchanged. The amendments in this ASU are effective for fiscal years beginning after December 15, 2018 and interim periods within those fiscal years. Early application is permitted for all entities. ASU 2016-02 requires a modified retrospective approach for all leases existing at, or entered into after, the date of initial application, with an option to elect to use certain transition relief. The Company is currently evaluating the impact of this new standard on its consolidated financial statements.

In August 2014, the FASB issued ASU 2014-15, which requires management of public and private companies to evaluate whether there is substantial doubt about the entity’s ability to continue as a going concern within one year after the date that the financial statements are issued. Management’s evaluation should be based on relevant conditions and events that are known and reasonably knowable at the date that the financial statements are issued. If conditions or events raise substantial doubt about an entity’s ability to continue as a going concern, and substantial doubt is not alleviated after consideration of management’s plans, an entity should include a statement in the footnotes indicating that there is substantial doubt about the entity’s ability to continue as a going concern within one year after the date that the financial statements are issued. The new standard is effective for annual periods ending after December 15, 2016, and for annual periods and interim periods thereafter. Early application is permitted. If this standard had been adopted as of March 31, 2016 and applied to these financial statements, the Company believes that there would be no significant impact to its disclosure. However, the Company faces certain risks and uncertainties as further described in Note 1, “Nature of Organization and Operations” that could affect this conclusion.

In May 2014, the FASB issued a new revenue recognition standard which amends revenue recognition principles and provides a single, comprehensive set of criteria for revenue recognition within and across all industries. The new standard provides a five step framework whereby revenue is recognized when promised goods or services are transferred to a customer at an amount that reflects the consideration to which the entity expects to be entitled in exchange for those goods or services. The standard also requires enhanced disclosures pertaining to revenue recognition in both interim and annual periods. In August 2015, the FASB deferred the effective date of the new revenue standard from January 1, 2017 to January 1, 2018. Early adoption is permitted any time after the original effective date, which for us is January 1, 2017. The standard allows for adoption using a full retrospective method or a modified retrospective method. The Company is currently evaluating the timing, method of adoption and the expected impact that the standard could have on our consolidated financial statements and related disclosures.

Segment Information

Operating segments are defined as components of an enterprise about which separate discrete information is available for evaluation by the chief operating decision maker, or decision-making group, in deciding how to allocate resources and in assessing performance. The Company views its operations and manages its business in one operating segment, which is the business of developing and commercializing proprietary therapeutics based on HIF biology.

Use of Estimates

The preparation of financial statements in conformity with U.S. GAAP requires management to make estimates and assumptions that affect the reported amounts of assets and liabilities and disclosure of contingent assets and liabilities at the date of the financial statements and the reported amounts of revenues and expenses during the reporting period. Actual results may differ from those estimates. Management considers many factors in selecting appropriate financial accounting policies and controls, and in developing the estimates and assumptions that are used in the preparation of these financial statements. Management must apply significant judgment in this process. In addition, other factors may affect estimates, including: expected business and operational changes, sensitivity and volatility associated with the assumptions used in developing estimates, and whether historical trends are expected to be representative of future trends. The estimation process often may yield a range of potentially reasonable estimates of the ultimate future outcomes, and management must select an amount that falls within that range of reasonable estimates. Estimates are used in the following areas, among others: prepaid and accrued research and development expense, stock-based compensation expense, accrued expenses and income taxes.

Prior to the IPO, the Company utilized significant estimates and assumptions in determining the fair value of its common stock. The Company granted stock options at exercise prices not less than the fair market value of its common stock as determined by the Board of Directors contemporaneously at the date such grants were made, with input from management. Prior to the Company’s IPO in March 2014, the fair value of common stock at the grant date was adjusted in connection with the Company’s retrospective fair value assessment for financial reporting purposes. Accordingly, the Board of Directors determined the estimated fair value of the Company’s common stock based on a number of objective and subjective factors, including external market conditions affecting the biotechnology industry sector and the prices at which the Company sold shares of preferred stock, the superior rights and preferences of securities senior to the Company’s common stock at the time and the likelihood of achieving a liquidity event, such as an IPO or sale of the Company.

Cash and Cash Equivalents

Cash and cash equivalents consist of all cash on hand, deposits and funds invested in available-for-sale securities with original maturities of three months or less at the time of purchase. At March 31, 2016, the Company’s cash is primarily in money market funds. The Company may maintain balances with its banks in excess of federally insured limits.

Investments

Management determines the appropriate classification of securities at the time of purchase and reevaluates such designation as of each balance sheet date. Currently, the Company classifies all securities as available-for-sale which are included in current assets as they are intended to fund current operations. The Company carries available-for-sale securities at fair value. The Company conducts periodic reviews to identify and evaluate each investment that has an unrealized loss, in accordance with the meaning of other-than-temporary impairment and its application to certain investments. When assessing whether a decline in the fair value of a security is other-than-temporary, the Company considers the fair market value of the security, the duration of the security’s decline, and prospects for the underlying business. Based on these considerations, the Company did not identify any other-than-temporary unrealized losses at March 31, 2016. Unrealized losses on available-for-sale securities that are determined to be temporary, and not related to credit loss, are recorded in accumulated other comprehensive loss, a component of stockholders’ equity. The amortized cost of debt securities in this category reflects amortization of premiums and accretion of discounts to maturity computed under the effective interest method. The Company includes this amortization in the caption “Interest income, net” within the Consolidated Statements of Operations and Comprehensive Loss. The Company also includes in net investment income, realized gains and losses and declines in value determined to be other than temporary. The Company bases the cost of securities sold upon the specific identification method, and includes interest and dividends on securities in interest income.

Revenue Recognition

To date, the Company has not generated any revenue from the sales of products or other means. For the foreseeable future, the Company expects substantially all of its revenues will be generated from its collaboration with Mitsubishi Tanabe (see Note 9) and any other collaborations the Company may enter into.

The Company will recognize revenue in accordance with ASC Topic 605, Revenue Recognition (ASC 605). Accordingly, revenue will be recognized for each unit of accounting when all of the following criteria are met:

Persuasive evidence of an arrangement exists;

Delivery has occurred or services have been rendered;

The seller’s price to the buyer is fixed or determinable; and

Collectability is reasonably assured.

Amounts received prior to satisfying the revenue recognition criteria are recorded as deferred revenue. Amounts expected to be recognized as revenue within the 12 months following the balance sheet date are classified in current liabilities. Amounts not expected to be recognized as revenue within the 12 months following the balance sheet date are classified as deferred revenue, net of current portion.

The Company evaluates multiple‑element arrangements based on the guidance in ASC Topic 605‑25, Revenue Recognition Multiple‑Element Arrangements (ASC 605‑25). Pursuant to the guidance in ASC 605‑25, the Company evaluates multiple‑element arrangements to determine (1) the deliverables included in the arrangement and (2) whether the individual deliverables represent separate units of accounting or whether they must be accounted for as a combined unit of accounting. This evaluation involves subjective determinations and requires the Company to make judgments about the individual deliverables and whether such deliverables are separable from the other aspects of the contractual relationship. Deliverables are considered separate units of accounting provided that the delivered item has value to the customer on a standalone basis and, if the arrangement includes a general right of return relative to the delivered item, delivery or performance of the undelivered item is considered probable and substantially in the Company’s control. In assessing whether an item has standalone value, the Company considers factors such as the research, development, manufacturing and commercialization capabilities of the collaboration partner and the availability of the associated expertise in the general marketplace. In addition, the Company considers whether the collaboration partner can use a deliverable for its intended purpose without the receipt of the remaining deliverable, whether the value of the deliverable is dependent on the undelivered item and whether there are other vendors that can provide the undelivered items.

The consideration received under the arrangement that is fixed or determinable is then allocated among the separate units of accounting using the relative selling price method. The Company determines the estimated selling price for units of accounting within each arrangement using vendor‑specific objective evidence (VSOE) of selling price, if available, third‑party evidence (TPE) of selling price if VSOE is not available, or best estimate of selling price (BESP) if neither VSOE nor TPE is available. Determining the BESP for a unit of accounting requires significant judgment. In developing the BESP for a unit of accounting, the Company considers applicable market conditions and relevant entity‑specific factors, including factors that were contemplated in negotiating the agreement with the customer and estimated costs. The Company validates the BESP for units of accounting by evaluating whether changes in the key assumptions used to determine the BESP will have a significant effect on the allocation of arrangement consideration between multiple units of accounting.

The Company will recognize arrangement consideration allocated to each unit of accounting when all of the revenue recognition criteria in ASC 605 are satisfied for that particular unit of accounting. In the event that a deliverable does not represent a separate unit of accounting, the Company will recognize revenue from the combined unit of accounting over the Company’s contractual or estimated performance period for the undelivered elements, which is typically the term of the Company’s research and development obligations. If there is no discernible pattern of performance or objectively measurable performance measures do not exist, then the Company will recognize revenue under the arrangement on a straight‑line basis over the period the Company is expected to complete its performance obligations. Conversely, if the pattern of performance in which the service is provided to the customer can be determined and objectively measurable performance measures exist, then the Company recognizes revenue under the arrangement using the proportional performance method. Revenue to be recognized is limited to the lesser of the cumulative amount of payments received or the cumulative amount of revenue earned, as determined using the straight‑line method or proportional performance method, as applicable, as of the period ending date.

At the inception of an arrangement that includes milestone payments, the Company evaluates whether each milestone is substantive and at risk to both parties on the basis of the contingent nature of the milestone. This evaluation includes an assessment of whether: (1) the consideration is commensurate with either the Company’s performance to achieve the milestone or the enhancement of the value of the delivered item(s) as a result of a specific outcome resulting from its performance to achieve the milestone, (2) the consideration relates solely to past performance and (3) the consideration is reasonable relative to all of the deliverables and payment terms within the arrangement. The Company evaluates factors such as the scientific, clinical, regulatory, commercial, and other risks that must be overcome to achieve the particular milestone and the level of effort and investment required to achieve the particular milestone in making this assessment. There is considerable judgment involved in determining whether a milestone satisfies all of the criteria required to conclude that a milestone is substantive. Milestones that are not considered substantive are recognized as earned if there are no remaining performance obligations or over the remaining period of performance, assuming all other revenue recognition criteria are met.

Options are considered substantive if, at the inception of the arrangement, the Company is at risk as to whether the collaboration partner will choose to exercise the option. Factors that the Company considers in evaluating whether an option is substantive include the cost to exercise the option, the overall objective of the arrangement, the benefit the collaborator might obtain from the arrangement without exercising the option and the likelihood the option will be exercised. When an option is considered substantive, the Company does not consider the option or item underlying the option to be a deliverable at the inception of the arrangement and the associated option fees are not included in allocable consideration, assuming the option is not priced at a significant and incremental discount. Conversely, when an option is not considered substantive, the Company would consider the option, including other deliverables contingent upon the exercise of the option, to be a deliverable at the inception of the arrangement and a corresponding amount would be included in allocable arrangement consideration. In addition, if the price of the option includes a significant incremental discount, the discount would be included as a deliverable at the inception of the arrangement.

The Company will recognize royalty revenue in the period of sale of the related product(s), based on the underlying contract terms, provided that the reported sales are reliably measurable and the Company has no remaining performance obligations, assuming all other revenue recognition criteria are met.

Patents

Costs incurred in connection with the application for and issuance of patents are expensed as incurred.

Income Taxes

Income taxes are recorded in accordance with ASC Topic 740, Income Taxes (ASC 740), which provides for deferred taxes using an asset and liability approach. The Company recognizes deferred tax assets and liabilities for the expected future tax consequences of events that have been included in the financial statements or tax returns. Deferred tax assets and liabilities are determined based on the difference between the financial statement and tax bases of assets and liabilities using enacted tax rates in effect for the year in which the differences are expected to reverse. Valuation allowances are provided, if, based upon the weight of available evidence, it is more likely than not that some or all of the deferred tax assets will not be realized.

The Company accounts for uncertain tax positions in accordance with the provisions of ASC 740. When uncertain tax positions exist, the Company recognizes the tax benefit of tax positions to the extent that the benefit will more likely than not be realized. The determination as to whether the tax benefit will more likely than not be realized is based upon the technical merits of the tax position, as well as consideration of the available facts and circumstances. As of March 31, 2016 and 2015, the Company does not have any significant uncertain tax positions. The Company recognizes interest and penalties related to uncertain tax positions in income tax expense.

Stock-Based Compensation

The Company accounts for its stock-based compensation awards in accordance with ASC Topic 718, Compensation—Stock Compensation (ASC 718). ASC 718 requires all stock-based payments to employees, including grants of employee stock options, restricted stock, restricted stock units, or RSUs, and modifications to existing stock awards, to be recognized in the statements of operations and comprehensive loss based on their fair values. The Company accounts for stock-based awards to non-employees in accordance with ASC Topic 505-50, Equity-Based Payments to Non-Employees (ASC 505-50), which requires the fair value of the award to be re-measured at fair value until a performance commitment is reached or counterparty performance is complete. The Company’s stock-based awards are comprised of stock options, shares of restricted stock and shares of common stock. The Company estimates the fair value of options granted using the Black-Scholes option pricing model. The Company uses the quoted market price of comparable public companies to determine the fair value of restricted stock awards and common stock awards.

The Black-Scholes option pricing model requires the input of certain subjective assumptions, including (a) the expected stock price volatility, (b) the calculation of expected term of the award, (c) the risk-free interest rate and (d) expected dividends. Due to the lack of company-specific historical and implied volatility data for trading the Company’s stock in the public market, the Company has based its estimate of expected volatility on the historical volatility of a group of similar companies that are publicly traded. The historical volatility is calculated based on a period of time commensurate with the expected term assumption. The computation of expected volatility is based on the historical volatility of a representative group of companies with similar characteristics to the Company, including stage of product development and life science industry focus. The Company is in the product development stage with no revenue and the representative group of companies has certain similar characteristics to the Company. The Company believes the group selected has sufficient similar economic and industry characteristics, and includes companies that are most representative of the Company. The Company uses the simplified method as prescribed by the SEC Staff Accounting Bulletin No. 107, Share-Based Payment, to calculate the expected term for options granted to employees as it does not have sufficient historical exercise data to provide a reasonable basis upon which to estimate the expected term. The expected term is applied to the stock option grant group as a whole, as the Company does not expect substantially different exercise or post-vesting termination behavior among its employee population. For options granted to non-employees, the Company utilizes the contractual term of the arrangement as the basis for the expected term assumption. The risk-free interest rate is based on a treasury instrument whose term is consistent with the expected life of the stock options. The expected dividend yield is assumed to be zero as the Company has never paid dividends and has no current plans to pay any dividends on its common stock, which is similar to the Company’s peer group.

The Company’s stock-based awards are subject to either service- or performance-based vesting conditions. Compensation expense related to awards to employees with service-based vesting conditions is recognized on a straight-line basis based on the grant date fair value over the associated service period of the award, which is generally the vesting term. Consistent with the guidance in ASC 505- 50, compensation expense related to awards to non-employees with service-based vesting conditions is recognized on a straight-line basis based on the then-current fair value at each financial reporting date prior to the measurement date over the associated service period of the award, which is generally the vesting term. Compensation expense related to awards to employees with performance-based vesting conditions is recognized based on the grant date fair value over the requisite service period using the accelerated attribution method to the extent achievement of the performance condition is probable. Consistent with the guidance in ASC 505-50, compensation expense related to awards to non-employees with performance-based vesting conditions is recognized based on the then-current fair value at each financial reporting date prior to the measurement date over the requisite service period using the accelerated attribution method to the extent achievement of the performance condition is probable.

The Company is also required to estimate forfeitures at the time of grant, and revise those estimates in the subsequent periods if actual forfeitures differ from its estimates. The Company uses historical data to estimate pre-vesting forfeitures and record stock-based compensation expense only for those awards that are expected to vest. To the extent that actual forfeitures differ from the Company’s estimates, the difference is recorded as a cumulative adjustment in the period the estimates were revised. Stock-based compensation expense recognized in the condensed consolidated financial statements is based on awards that are ultimately expected to vest.

Fair Value of Financial Instruments

The Company is required to disclose information on all assets and liabilities reported at fair value that enables an assessment of the inputs used in determining the reported fair values. ASC Topic 820, Fair Value Measurements and Disclosures (ASC 820), establishes a hierarchy of inputs used in measuring fair value that maximizes the use of observable inputs and minimizes the use of unobservable inputs by requiring that the observable inputs be used when available.

Observable inputs are inputs that market participants would use in pricing the asset or liability based on market data obtained from sources independent of the Company. Unobservable inputs are inputs that reflect the Company’s assumptions about the inputs that market participants would use in pricing the asset or liability, and are developed based on the best information available in the circumstances. The fair value hierarchy applies only to the valuation inputs used in determining the reported fair value of the investments, and is not a measure of the investment credit quality. The three levels of the fair value hierarchy are described below:

Level 1 – Valuations based on unadjusted quoted prices in active markets for identical assets or liabilities that the Company has the ability to access at the measurement date.

Level 2 – Valuations based on quoted prices for similar assets or liabilities in markets that are not active, or for which all significant inputs are observable, either directly or indirectly.

Level 3 – Valuations that require inputs that reflect the Company’s own assumptions that are both significant to the fair value measurement and unobservable.

To the extent that valuation is based on models or inputs that are less observable or unobservable in the market, the determination of fair value requires more judgment. Accordingly, the degree of judgment exercised by the Company in determining fair value is greatest for instruments categorized in Level 3. A financial instrument’s level within the fair value hierarchy is based on the lowest level of any input that is significant to the fair value measurement.

Items measured at fair value on a recurring basis include short-term investments (see Note 4). The carrying amounts of prepaid expenses and other current assets, accounts payable and accrued expenses approximate their fair values due to their short-term maturities. The rate implicit within the Company’s capital lease obligation approximates market interest rates.

Concentrations of Credit Risk and Off-Balance Sheet Risk

Cash and investments are the only financial instruments that potentially subject the Company to concentrations of credit risk. The Company maintains its cash with high quality, accredited financial institutions and, accordingly, such funds are subject to minimal credit risk. The Company has no significant off-balance sheet concentrations of credit risk, such as foreign currency exchange contracts, option contracts or other hedging arrangements.

Net Loss per Share

Basic net loss per share is calculated by dividing net loss attributable to common stockholders by the weighted-average shares outstanding during the period, without consideration for common stock equivalents. Diluted net loss per share is calculated by adjusting weighted-average shares outstanding for the dilutive effect of common stock equivalents outstanding for the period, determined using the treasury-stock method. For purposes of the diluted net loss per share calculation, preferred stock, stock options, unvested restricted stock and RSUs are considered to be common stock equivalents, but have been excluded from the calculation of diluted net loss per share, as their effect would be anti-dilutive for all periods presented. Therefore, basic and diluted net loss per share were the same for all periods presented.

Property and Equipment

Property and equipment is stated at cost, less accumulated depreciation. Assets under capital lease are included in property and equipment. Property and equipment is depreciated using the straight-line method over the estimated useful lives of the assets, generally three to seven years. Such costs are periodically reviewed for recoverability when impairment indicators are present. Such indicators include, among other factors, operating losses, unused capacity, market value declines and technological obsolescence. Recorded values of asset groups of equipment that are not expected to be recovered through undiscounted future net cash flows are written down to current fair value, which generally is determined from estimated discounted future net cash flows (assets held for use) or net realizable value (assets held for sale).

The following is the summary of property and equipment and related accumulated depreciation as of March 31, 2016 and December 31, 2015.

	Useful Life	March 31, 2016			December 31, 2015
		(in thousands)
Computer equipment and software	3	$	397		$	300
Furniture and fixtures	5		244			243
Equipment	7		50			50
Leasehold improvements	Shorter of the useful life or remaining lease term (3 years)		81			70
Office equipment under capital lease	3		36			24
			808			687
Less accumulated depreciation			(179	)		(147	)
Net property and equipment		$	629		$	540

Depreciation expense, including expense associated with assets under capital leases, was approximately $32,000 and $19,000 for the three months ended March 31, 2016 and 2015, respectively.

Available for Sale Securities

3 Months Ended

Mar. 31, 2016

Investments Debt And Equity Securities [Abstract]

Available for Sale Securities

3. Available for sale securities

Available for sale securities at March 31, 2016 and December 31, 2015 consist of the following:

			Gross		Gross
			Unrealized		Unrealized
	Amortized Cost		Gains		Losses			Fair Value
	(in thousands)
March 31, 2016
Cash and cash equivalents	$	43,439	$	—	$	—		$	43,439
Available for sale securities:
Certificates of deposit	$	22,326	—		—			$	22,326
U.S. Government debt securities		128,548		14		(36	)		128,526
Corporate debt securities		22,739		8		(13	)		22,734
Total available for sale securities	$	173,613	$	22	$	(49	)	$	173,586
Total cash, cash equivalents, and available for sale securities	$	217,052	$	22	$	(49	)	$	217,025

The estimated fair value of the Company’s available for sale securities balance at March 31, 2016, by contractual maturity, is as follows:

Due in one year or less	$	122,978
Due after one year		50,608
Total available for sale securities	$	173,586

			Gross		Gross
			Unrealized		Unrealized
	Amortized Cost		Gains		Losses			Fair Value
	(in thousands)
December 31, 2015
Cash and cash equivalents	$	49,778	$	—	$	—		$	49,778
Available for sale securities:
Certificates of deposit	$	21,505	—			—		$	21,505
U.S. Government debt securities		46,461	—			(185	)		46,276
Corporate debt securities		20,944		1		(50	)		20,895
Total available for sale securities	$	88,910	$	1	$	(235	)	$	88,676
Total cash, cash equivalents, and available for sale securities	$	138,688	$	1	$	(235	)	$	138,454

- Definition

The entire disclosure for investments in certain debt and equity securities.

+ References + Details

Fair Value of Financial Instruments

3 Months Ended

Mar. 31, 2016

Fair Value Disclosures [Abstract]

Fair Value of Financial Instruments

4. Fair Value of Financial Instruments

The Company utilizes a portfolio management company for the valuation of the majority of its investments. This company is an independent, third-party vendor recognized to be an industry leader with access to market information that obtains or computes fair market values from quoted market prices, pricing for similar securities, recently executed transactions, cash flow models with yield curves and other pricing models. For valuations obtained from the pricing service, the Company performs due diligence to understand how the valuation was calculated or derived, focusing on the valuation technique used and the nature of the inputs.

Based on the fair value hierarchy, the Company classifies its cash equivalents and marketable securities within Level 1 or Level 2. This is because the Company values its cash equivalents and marketable securities using quoted market prices or alternative pricing sources and models utilizing market observable inputs.

Assets measured or disclosed at fair value on a recurring basis as of March 31, 2016 are summarized below:

	Fair Value Measurements Using
	Level 1		Level 2		Level 3		Total
	(in thousands)
Assets:
Cash and cash equivalents	$	43,439	$	—	$	—	$	43,439
Certificates of deposit		—		22,326		—		22,326
U.S. Government debt securities		—		128,526		—		128,526
Corporate debt securities		—		22,734		—		22,734
	$	43,439	$	173,586	$	—	$	217,025

The Company’s corporate debt securities are all investment grade.

Assets measured or disclosed at fair value on a recurring basis as of December 31, 2015 are summarized below:

	Fair Value Measurements Using
	Level 1		Level 2		Level 3		Total
	(in thousands)
Assets:
Cash and cash equivalents	$	49,778	$	—	$	—	$	49,778
Certificates of deposit		—		21,505		—		21,505
U.S. Government debt securities		—		46,276		—		46,276
Corporate debt securities		—		20,895		—		20,895
	$	49,778	$	88,676	$	—	$	138,454

The Company had no assets or liabilities measured at fair value on a recurring basis using significant unobservable inputs (Level 3) at March 31, 2016 and December 31, 2015.

Investment securities are exposed to various risks such as interest rate, market and credit risks. Due to the level of risk associated with certain investment securities and the level of uncertainty related to changes in the value of investment securities, it is at least reasonably possible that changes in risks in the near term would result in material changes in the fair value of investments.

- Definition

The entire disclosure for the fair value of financial instruments (as defined), including financial assets and financial liabilities (collectively, as defined), and the measurements of those instruments as well as disclosures related to the fair value of non-financial assets and liabilities. Such disclosures about the financial instruments, assets, and liabilities would include: (1) the fair value of the required items together with their carrying amounts (as appropriate); (2) for items for which it is not practicable to estimate fair value, disclosure would include: (a) information pertinent to estimating fair value (including, carrying amount, effective interest rate, and maturity, and (b) the reasons why it is not practicable to estimate fair value; (3) significant concentrations of credit risk including: (a) information about the activity, region, or economic characteristics identifying a concentration, (b) the maximum amount of loss the entity is exposed to based on the gross fair value of the related item, (c) policy for requiring collateral or other security and information as to accessing such collateral or security, and (d) the nature and brief description of such collateral or security; (4) quantitative information about market risks and how such risks are managed; (5) for items measured on both a recurring and nonrecurring basis information regarding the inputs used to develop the fair value measurement; and (6) for items presented in the financial statement for which fair value measurement is elected: (a) information necessary to understand the reasons for the election, (b) discussion of the effect of fair value changes on earnings, (c) a description of [similar groups] items for which the election is made and the relation thereof to the balance sheet, the aggregate carrying value of items included in the balance sheet that are not eligible for the election; (7) all other required (as defined) and desired information.

+ References + Details

Accrued Expenses

3 Months Ended

Mar. 31, 2016

Payables And Accruals [Abstract]

Accrued Expenses

5. Accrued Expenses

Accrued expenses are as follows:


	March 31, 2016		December 31, 2015
	(in thousands)
Accrued clinical	$	6,998	$	4,536
Professional fees		1,043		647
Accrued bonus		679		2,178
Accrued vacation		425		310
Accrued severance		319	—
Accrued payroll		218		518
Other		1,312		1,366
Total accrued expenses	$	10,994	$	9,555

In August 2014, the Company entered into a separation agreement with an employee. The Company recorded the expense and liability associated with the separation agreement ratably over the period from August 5, 2014 through December 31, 2015 because the severance payments were subject to continued service and forfeiture until December 31, 2015. During the first quarter of 2015, the Company recorded severance expense in the amount of approximately $0.1 million, which was recorded to research and development expense. Payments under this separation agreement were paid out through December 2015.

In January 2016, the Company entered into separation agreements with two employees. During the first quarter of 2016, the Company recorded severance expense in the amount of $0.4 million, of which $0.2 million was recorded to general and administrative expense and $0.2 million was recorded to research and development expense. During the first quarter of 2016, approximately $0.1 million was paid out of the severance accrual.

Stockholders' Equity

3 Months Ended

Mar. 31, 2016

Equity [Abstract]

Stockholders' Equity

6. Stockholders’ Equity

Authorized and Outstanding Capital Stock

As of December 31, 2014, the authorized capital stock of the Company included 175,000,000 shares of common stock, par value $0.00001 per share, of which 37,946,010 and 30,662,218 shares are issued and outstanding at March 31, 2016 and December 31, 2015, respectively; and 25,000,000 shares of undesignated preferred stock, par value $0.00001 per share, of which 0 shares are issued and outstanding at March 31, 2016 and December 31, 2015.

Equity Plans

On February 28, 2014, the Company’s Board of Directors adopted its 2014 Incentive Plan (2014 Plan) and its 2014 Employee Stock Purchase Plan (ESPP), which were subsequently approved by its stockholders and became effective upon the closing of the Company’s IPO on March 25, 2014. The 2014 Plan replaces the 2008 Equity Incentive Plan (2008 Plan), however, any options or awards outstanding under the 2008 Plan at the time of adoption of the 2014 Plan remain outstanding and effective.

The 2014 Plan allows for the granting of stock options, stock appreciation rights, or SARs, restricted stock, unrestricted stock, stock units, performance awards and other awards convertible into or otherwise based on shares of our common stock. Dividend equivalents may also be provided in connection with an award under the 2014 Plan. The Company’s employees, officers, directors and consultants and advisors are eligible to receive awards under the 2014 Plan. The Company initially reserved 1,785,000 shares of its common stock for the issuance of awards under the 2014 Plan. The 2014 Plan provides that the number of shares reserved and available for issuance under the 2014 Plan will automatically increase annually on January 1^st of each calendar year, by an amount equal to three percent (3%) of the number of shares of stock outstanding on a fully diluted basis as of the close of business on the immediately preceding December 31^st(2014 Plan Evergreen Provision). The Company’s Board of Directors may act prior to January 1^st of any year to provide that there will be no automatic increase in the number of shares available for grant under the 2014 Plan for that year (or that the increase will be less than the amount that would otherwise have automatically been made). Subject to adjustment, no more than 1,131,937 shares of our common stock may be delivered in satisfaction of incentive stock options awarded under the 2014 Plan. During the first three months of 2016, the Company granted 693,775 stock options to employees, 382,338 RSUs to employees and no equity grants were made to directors under the 2014 Plan.

The ESPP authorizes the initial issuance of up to a total of 262,500 share of the Company’s common stock to participating employees. The maximum aggregate number of shares of common stock available for purchase pursuant to the exercise of options granted under the ESPP will be the lesser of (a) 262,500 shares, increased on each anniversary of the adoption of the ESPP by one percent (1%) of the total shares of common stock then outstanding and (b) 739,611 shares (which is equal to five percent (5%) of the total shares of common stock outstanding on the date of the adoption of the ESPP on a fully diluted, as converted basis (ESPP Evergreen Provision). Under the ESPP, each offering period is six months, at the end of which employees may purchase shares of common stock through payroll deductions made over the term of the offering. The per-share purchase price at the end of each offering period is equal to the lesser of 85% of the closing price of our common stock at the beginning or end of the offering period.

Shares Reserved for Future Issuance

The Company has reserved for future issuance the following number of shares of common stock:


	March 31, 2016		December 31, 2015
Unvested restricted common stock		189,273		216,716
Common stock options and RSU's outstanding		3,112,081		2,231,057
Shares available for issuance under the 2014 Plan ⁽¹⁾		1,390,718		1,318,732
Shares available for issuance under the ESPP ⁽²⁾		819,734		440,304
Total		5,511,806		4,206,809

⁽¹⁾

On January 1, 2016, the shares reserved for future grants under the 2014 Plan increased by 986,800 shares pursuant to the 2014 Plan Evergreen Provision.

⁽²⁾

On February 28, 2016, the shares reserved for future issuance under the ESPP increased by 379,430 shares pursuant to the ESPP Evergreen Provision.

Stock-Based Compensation

Stock Options

On February 22, 2016, the Company issued 624,275 stock options to employees. Options granted by the Company vest over periods of between 12 and 48 months. Options vest in installments of (i) 25% at the one year anniversary and (ii) in either 36 or 48 equal monthly or 12 equal quarterly installments beginning in the thirteenth month after the initial Vesting Commencement Date (as defined) or grant date, subject to the employee’s continuous service with the Company. Options generally expire ten years after the date of grant. The Company recorded approximately $1.1 million of stock-based compensation expense related to stock options during the first quarter of 2016.

Restricted Stock

On December 23, 2013, the Company issued 450,224 shares of restricted stock to employees and 79,067 shares of restricted stock to non-employees at a grant date fair value of $7.42 per share. The aggregate grant date fair value for the shares of restricted stock issued on December 23, 2013 totaled approximately $3.9 million. The awards of restricted stock contained a performance condition wherein vesting is contingent upon the Company’s consummation of a liquidity event, as defined, prior to the fifth anniversary of the date of grant. Certain of the awards of restricted stock have a requisite service period that was complete upon grant. The remainder of the awards of restricted stock have a requisite service period of four years whereby the award vests 25% on the one year anniversary of the Vesting Commencement Date (as defined), then ratably on the first day of each calendar quarter for 12 quarters, subject to continuous service by the individual and achievement of the performance target. Due to the nature of the performance condition, the Company had concluded that the performance condition was not probable of achievement and therefore, recognition of compensation cost had been deferred until the occurrence of a liquidity event, as defined. Compensation expense related to the restricted stock awards is being recognized over the associated requisite service period which commenced on March 25, 2014. The Company reversed approximately $14,000 of stock-based compensation expense related to restricted stock during the first quarter of 2016 as a result of mark to market adjustments related to non-employees.

Restricted Stock Units

On February 22, 2016, the Company issued 382,338 RSUs to employees. The RSUs vest 100% on the third year anniversary of the grant date. Total stock-compensation expense to be recognized over the life of the RSUs is $2.9 million and will be recognized on a straight-line basis over the vesting period. The Company recorded approximately $0.1 million of stock-based compensation expense related to the RSUs during the first quarter of 2016.

ESPP

The first offering period under the ESPP opened on January 2, 2015. There were no shares issued during the first quarter of 2016. The Company recorded approximately $31,000 of stock-based compensation expense related to ESPP during the first quarter of 2016.

Compensation Expense Summary

The Company has recognized the following compensation cost related to share-based awards:

	Three months ended
	March 31, 2016		March 31, 2015
	(in thousands)
Research and development	$	392	$	565
General and administrative		859		563
Total	$	1,251	$	1,128

Compensation expense by type of award:

	Three months ended
	March 31, 2016			March 31, 2015
	(in thousands)
Stock options	$	1,128		$	699
Restricted stock		(14	)		400
Restricted stock units		106			8
Employee stock purchase plan		31			21
Total	$	1,251		$	1,128

- Definition

The entire disclosure for shareholders' equity comprised of portions attributable to the parent entity and noncontrolling interest, including other comprehensive income. Includes, but is not limited to, balances of common stock, preferred stock, additional paid-in capital, other capital and retained earnings, accumulated balance for each classification of other comprehensive income and amount of comprehensive income.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 235
-SubTopic 10
-Section S99
-Paragraph 1
-Subparagraph (SX 210.4-08.(d),(e))
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Reference 3: http://www.xbrl.org/2003/role/presentationRef
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-Topic 310
-SubTopic 10
-Section S99
-Paragraph 2
-Subparagraph (SAB TOPIC 4.E)
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Reference 4: http://www.xbrl.org/2003/role/presentationRef
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-URI http://asc.fasb.org/extlink&oid=6928386&loc=d3e21488-112644

Reference 5: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 210
-SubTopic 10
-Section S99
-Paragraph 1
-Subparagraph (SX 210.5-02.29-31)
-URI http://asc.fasb.org/extlink&oid=6877327&loc=d3e13212-122682

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-Name Accounting Standards Codification
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Reference 7: http://www.xbrl.org/2003/role/presentationRef
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Reference 8: http://www.xbrl.org/2003/role/presentationRef
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-Name Accounting Standards Codification
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-Paragraph 1
-Subparagraph (SX 210.3-04)
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Reference 9: http://www.xbrl.org/2003/role/presentationRef
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-Name Accounting Standards Codification
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-Publisher FASB
-Name Accounting Standards Codification
-Topic 505
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-Paragraph 3
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Reference 11: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Glossary Preferred Stock
-URI http://asc.fasb.org/extlink&oid=6521494

Reference 12: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 505
-SubTopic 10
-Section 50
-Paragraph 2
-URI http://asc.fasb.org/extlink&oid=6928386&loc=d3e21463-112644

Reference 13: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 505
-SubTopic 10
-Section 50
-Paragraph 11
-URI http://asc.fasb.org/extlink&oid=6928386&loc=d3e21564-112644

Reference 14: http://www.xbrl.org/2003/role/presentationRef
-Publisher SEC
-Name Staff Accounting Bulletin (SAB)
-Number Topic 4
-Section E

Reference 15: http://www.xbrl.org/2003/role/presentationRef
-Publisher SEC
-Name Staff Accounting Bulletin (SAB)
-Number Topic 4
-Section C

+ Details

Income Taxes	3 Months Ended
Income Taxes	Mar. 31, 2016
Income Tax Disclosure [Abstract]
Income Taxes	7. Income Taxes Deferred tax assets and deferred tax liabilities are recognized based on temporary differences between the financial reporting and tax basis of assets and liabilities using statutory rates. A valuation allowance is recorded against deferred tax assets if it is more likely than not that some or all of the deferred tax assets will not be realized. There were no significant income tax provisions or benefits for the three months ended March 31, 2016 and 2015. Due to the uncertainty surrounding the realization of the favorable tax attributes in future tax returns, the Company has recorded a full valuation allowance against the Company’s otherwise recognizable net deferred tax assets.

Commitments and Contingencies

3 Months Ended

Mar. 31, 2016

Commitments And Contingencies Disclosure [Abstract]

Commitments and Contingencies

8. Commitments and Contingencies

In December 2013, the Company entered into a three-year lease for 6,837 square feet of office space in Cambridge, Massachusetts. The lease has monthly payments of approximately $31,000 for the first twelve months, with annual rent escalation thereafter, and provides a rent abatement of approximately $31,000 for the first full calendar month of the lease term. The lease term commenced and rental payments began in January 2014. The Company records a deferred lease obligation which represents the cumulative difference between actual facility lease payments and lease expense recognized ratably over the lease period, which is included in other liabilities. In accordance with the lease, the Company entered into a cash-collateralized irrevocable standby letter of credit in the amount of $125,345, naming the landlord as beneficiary.

In December 2014, the Company entered into a First Amendment to Lease, or the First Amendment, for additional office space contiguous to its current office space in Cambridge, Massachusetts. The First Amendment included leasing an additional 8,530 square feet of office space, or the Expansion Space, with an occupancy date of March 13, 2015. The First Amendment provided for additional monthly lease payments of approximately $45,000 for 8,530 square feet for the first twelve months and provided for annual rent escalations thereafter. The monthly rent on the existing 6,837 square feet remained at approximately $32,000 through December 31, 2016, the expiration of the lease. The First Amendment included a contribution from the landlord for leasehold improvements in the amount of approximately $100,000 which was accounted for as a deferred lease incentive and reduction in monthly rent expense over the term of the lease. The Company recorded an additional deferred lease obligation for the Expansion Space which represented the cumulative difference between actual facility lease payments and lease expense recognized ratably over the lease period. In connection with the Amendment, the Company paid an additional cash security deposit to the landlord of $179,130.

In November 2015, the Company entered into a Second Amendment to Lease, or the Second Amendment, amending the Lease to extend its existing lease for 16,222 square feet on the 11^th floor, which was due to expire on December 31, 2016, and lease an additional 23,189 square feet of office space on the 11^th and 14^th floors. Monthly lease payments for the existing 16,222 square feet on the 11^th floor remain unchanged until December 31, 2016 and will be approximately $107,000 per month commencing on May 1, 2017. The new space leased by the Company under the Second Amendment includes (i) 3,066 square feet on the 14^th floor which was delivered on December 31, 2015 and additional monthly lease payments of approximately $20,000 commencing on April 1, 2016, (ii) 16,739 square feet on the 14^th floor which was delivered on April 11, 2016 and additional monthly lease payments of approximately $110,000 commencing September 11, 2016 and (iii) 3,384 square feet on the 11^th floor with an estimated delivery date of January 1, 2017 and additional monthly lease payments of approximately $23,000 commencing May 1, 2017. The above rents are subject to annual rent escalations, commencing on either February 1, 2017 or February 1, 2018 depending upon the delivery date of each respective suite. The Second Amendment includes a contribution from the landlord for leasehold improvements for both the new leased space and the Company’s existing premises. The contribution from the landlord is being accounted for as a deferred lease incentive and reduction in monthly rent expense over the term of the lease. The term of the lease, as amended, expires on the later of August 31, 2026 or 10 years after the landlord delivers the entirety of the 14^th floor to the Company. In connection with the Second Amendment, the Company paid an additional cash security deposit to the landlord of $976,382. Total cash security deposits of $1,280,857 and $304,475 are included in other assets in the Company’s condensed consolidated balance sheets as of March 31, 2016 and December 31, 2015, respectively.

The Company recognizes rent expense for the space which it currently occupies under its initial lease, the First Amendment, and the Second Amendment, and records a deferred lease obligation representing the cumulative difference between actual facility lease payments and lease expense recognized ratably over the lease period, which is included in the Company’s condensed consolidated balance sheets as of March 31, 2016 and December 31, 2015. The Company will begin recognizing rent expense for the additional space not yet available at March 31, 2016 subsequent to taking possession of the space.

The Company leases office equipment under three year capital leases with payments commencing in February 2014, April 2015 and February 2016, respectively. The capital lease amounts are included in accrued expenses and other liabilities.

At March 31, 2016, the Company’s future minimum payments required under these leases are as follows:

	Operating		Capital
	Lease		Lease			Total
	(in thousands)
2016	$	2,140	$	9		$	2,149
2017		3,121		9			3,130
2018		3,121		5			3,126
2019		3,121		—			3,121
2020		3,121		—			3,121
Thereafter		17,685		—			17,685
Total	$	32,309		23		$	32,332
Less amount representing interest				(1	)
Present value of minimum lease payments at March 31, 2016			$	22

The Company recorded approximately $0.4 million and $0.1 million in rent expense for the three months ended March 31, 2016 and 2015, respectively.

Under the Company’s agreement with Quintiles, Inc., or Quintiles, to conduct the PRO₂TECT and INNO₂VATE programs, the total remaining contract costs as of March 31, 2016 were approximately $238.8 million. The estimated period of performance for the committed work with Quintiles is through the third quarter of 2019. The Company contracts with various other organizations to conduct research and development activities with remaining contract costs to the Company of approximately $13.7 million and 5.2 million at March 31, 2016 and December 31, 2015, respectively. The scope of the services under the research and development contracts can be modified and the contracts cancelled by the Company upon written notice. In some instances, the contracts may be cancelled by the third party upon written notice.

In September 2015, a purported securities class action lawsuit was filed against the Company, including its Chief Executive Officer, its Chief Financial Officer, and members of its Board of Directors, in the Business Litigation Section of the Suffolk County Superior Court of Massachusetts. The complaint is brought on behalf of an alleged class of those who purchased common stock of the Company pursuant or traceable to our IPO, and purports to allege claims arising under Sections 11, 12(a)(2) and 15 of the Securities Act of 1933, as amended. The complaint generally alleges that the defendants violated the federal securities laws by, among other things, making material misstatements or omissions concerning the Phase 2b clinical study of vadadustat. The complaint seeks, among other relief, unspecified compensatory damages, rescission of certain stock purchases, attorneys’ fees, and costs. In October 2015, we removed the case to the United States District Court for the District of Massachusetts, and the plaintiff filed a motion to remand the case back to the Business Litigation Section of the Suffolk County Superior Court of Massachusetts. The plaintiff’s motion to remand was granted in April 2016. The Company believes such claims are without merit and will engage in a vigorous defense of such litigation.

We have had a number of positive developments in our opposition and invalidity proceedings against FibroGen, Inc., or FibroGen. With regard to the opposition that we filed in Europe against FibroGen’s European Patent No. 1463823, or the ’823 patent, an oral proceeding took place March 8 and 9, 2016. Following the oral proceeding, the European Opposition Division ruled that the patent as granted did not meet the requirements for patentability under the European Patent Convention and, therefore, revoked the patent in its entirety. FibroGen has appealed that decision. Likewise, with regard to the invalidity proceeding that we filed in Japan against certain claims of FibroGen’s Japanese Patent No. 4804131, or the ’131 patent, which is the Japanese counterpart to the ’823 patent, the Japan Patent Office, or JPO, issued a preliminary decision finding all of the challenged claims to be invalid. FibroGen subsequently amended the claims and the JPO accepted the amendments. The resulting FibroGen Japanese ’131 patent does not cover vadadustat or any pyridine carboxamide compounds. To date, FibroGen has been unsuccessful in its attempts to obtain a patent in the United States covering the same claim scope as it obtained in Europe and Japan in the ’823 and ’131 patents. In the event FibroGen were to obtain such a patent in the United States, we may decide to challenge them like we have done in Europe and Japan.

On May 13, 2015, May 20, 2015 and July 6, 2015 we filed oppositions to FibroGen’s European Patent Nos. 2322153, 2322155, and 1633333, or the ’153 patent, the ’155 patent, and the ’333 patent, respectively, requesting the patents be revoked in their entirety. These related patents claim, among other things, various compounds that either stabilize HIFα or inhibit a HIF hydroxylase or a HIF prolyl hydroxylase for treating or preventing various conditions, including, inter alia, iron deficiency, microcytosis associated with iron deficiency, anemia of chronic disease, anemia wherein the subject has a transferrin saturation of less than 20%, anemia refractory to treatment with exogenously administered erythropoietin, or EPO, and microcytosis in microcytic anemia. Oppositions to the ’155 patent and to the ’153 patent were also filed by Glaxo Group Limited and by Bayer Intellectual Property GmbH, Bayer Pharma Aktiengesellschaft, and Bayer Animal Health GmbH. While, for the reasons set forth in our oppositions, we believe that the ’153 patent, the ’155 patent, and the ’333 patent should be revoked in their entireties, the ultimate outcomes of the oppositions remains uncertain. If the European Patent Office decides not to revoke the ’153 patent, the ’155 patent, or the ’333 patent in their entireties, or only certain claims of those patents, and any surviving claims are determined to encompass our intended use of our lead product candidate, we may not be able to commercialize our lead product candidate in the European Union for its intended use, which could materially adversely affect our business, operating results and financial condition.

The Company is not able to predict the outcome of the lawsuit described above or estimate the amount or range of any possible loss the Company might incur if the Company does not prevail in the final, non-appealable determination of this lawsuit. Therefore, we have not accrued any amounts in connection with this lawsuit.

Significant Agreements	3 Months Ended
Significant Agreements	Mar. 31, 2016
Organization Consolidation And Presentation Of Financial Statements [Abstract]
Significant Agreements	9. Significant Agreements Mitsubishi Tanabe Pharma Collaboration Agreement On December 11, 2015, the Company and Mitsubishi Tanabe Pharma Corporation, or MTPC, entered into a collaboration agreement (Collaboration Agreement) providing MTPC with exclusive development and commercialization rights to vadadustat (formerly known as AKB-6548), the Company’s product candidate for the treatment of anemia related to chronic kidney disease, in Japan and certain other Asian countries (collectively, the Territory). Pursuant to the Collaboration Agreement, MTPC will have an exclusive license to develop and commercialize vadadustat in the Territory. In addition, Akebia will supply vadadustat for both clinical and commercial use in the Territory. The countries included in the Territory are Japan, Taiwan, South Korea, Singapore, Malaysia, India, Indonesia, East Timor, Mongolia, the Philippines, Vietnam, Laos, Cambodia, Thailand, Brunei, Myanmar, Nepal, Sri Lanka, Bangladesh, Bhutan, Maldives, Palau and Tonga. In consideration for the exclusive license and other rights contained in the Collaboration Agreement, MTPC will make payments totaling up to $100.0 million to fund the vadadustat global Phase 3 program, including $40.0 million paid upon signing in January 2016. If Japanese patients are not included in either the global Phase 3 PRO₂TECT or INNO₂VATE programs, $20.0 million of the $40.0 million received would be used to fund further local development of vadadustat in Japan or be refunded to MTPC. Akebia is also eligible to receive up to approximately $250.0 million in additional milestone payments, based upon achievement of certain development and sales milestones as well as tiered royalty payments, from low teens up to twenty percent, on sales of vadadustat in the Territory. Akebia and MTPC will establish a joint steering committee to oversee development and commercialization of vadadustat in the Territory, including approval of any development or commercialization plans. Unless earlier terminated, the Collaboration Agreement will continue in effect on a country-by-country basis until the later of: expiration of the last-to-expire patent covering vadadustat in such country in the Territory; expiration of marketing or regulatory exclusivity in such country in the Territory; or ten (10) years after the first commercial sale of vadadustat in such country in the Territory. MTPC may terminate the Collaboration Agreement upon twelve (12) months’ notice at any time after the second anniversary of the effective date of the Collaboration Agreement. Either party may terminate the Collaboration Agreement upon the material breach of the other party that is not cured within a specified time period or upon the insolvency of the other party. As of March 31, 2016, there is approximately $40.0 million of deferred revenue related to the Company’s Collaboration Agreement, all of which is classified as long‑term in the accompanying condensed consolidated balance sheet. Revenue recognition for the Collaboration Agreement will commence once the Company begins to provide services in connection with the arrangement.

Employee Retirement Plan	3 Months Ended
Employee Retirement Plan	Mar. 31, 2016
Compensation And Retirement Disclosure [Abstract]
Employee Retirement Plan	10. Employee Retirement Plan During 2008, the Company established a retirement plan (the Plan) authorized by Section 401(k) of the Internal Revenue Code. In accordance with the Plan, all employees who have attained the age of 21 are eligible to participate in the Plan as of the first Entry Date, as defined, following their date of employment. Each employee can contribute a percentage of compensation up to a maximum of the statutory limits per year. Company contributions are discretionary and contributions in the amount of approximately $0.1 million and $0 were made during the three months ended March 31, 2016 and 2015, respectively.

Net Loss per Share

3 Months Ended

Mar. 31, 2016

Earnings Per Share [Abstract]

Net Loss per Share

11. Net Loss per Share

The shares in the table below were excluded from the calculation of diluted net loss per share, prior to the use of the treasury stock method, due to their anti-dilutive effect:

	Three months ended
	March 31, 2016		March 31, 2015
Outstanding stock options		2,706,718		1,935,099
Unvested restricted stock		189,273		345,019
Unvested restricted stock units		405,363		27,875
Total		3,301,354		2,307,993

Summary of Significant Accounting Policies (Policies)

3 Months Ended

Mar. 31, 2016

Accounting Policies [Abstract]

Basis of Presentation

Recent Accounting Pronouncements

Segment Information

Use of Estimates

Cash and Cash Equivalents

Investments

Revenue Recognition

Persuasive evidence of an arrangement exists;

Delivery has occurred or services have been rendered;

The seller’s price to the buyer is fixed or determinable; and

Collectability is reasonably assured.

Patents

Costs incurred in connection with the application for and issuance of patents are expensed as incurred.

Income Taxes

Stock-Based Compensation

Fair Value of Financial Instruments

Level 1 – Valuations based on unadjusted quoted prices in active markets for identical assets or liabilities that the Company has the ability to access at the measurement date.

Level 2 – Valuations based on quoted prices for similar assets or liabilities in markets that are not active, or for which all significant inputs are observable, either directly or indirectly.

Level 3 – Valuations that require inputs that reflect the Company’s own assumptions that are both significant to the fair value measurement and unobservable.

Concentrations of Credit Risk and Off-Balance Sheet Risk

Net Loss per Share

Property and Equipment

The following is the summary of property and equipment and related accumulated depreciation as of March 31, 2016 and December 31, 2015.

	Useful Life	March 31, 2016			December 31, 2015
		(in thousands)
Computer equipment and software	3	$	397		$	300
Furniture and fixtures	5		244			243
Equipment	7		50			50
Leasehold improvements	Shorter of the useful life or remaining lease term (3 years)		81			70
Office equipment under capital lease	3		36			24
			808			687
Less accumulated depreciation			(179	)		(147	)
Net property and equipment		$	629		$	540

Depreciation expense, including expense associated with assets under capital leases, was approximately $32,000 and $19,000 for the three months ended March 31, 2016 and 2015, respectively.

Summary of Significant Accounting Policies (Tables)

3 Months Ended

Mar. 31, 2016

Accounting Policies [Abstract]

Summary of Equipment and Related Accumulated Depreciation

The following is the summary of property and equipment and related accumulated depreciation as of March 31, 2016 and December 31, 2015.

	Useful Life	March 31, 2016			December 31, 2015
		(in thousands)
Computer equipment and software	3	$	397		$	300
Furniture and fixtures	5		244			243
Equipment	7		50			50
Leasehold improvements	Shorter of the useful life or remaining lease term (3 years)		81			70
Office equipment under capital lease	3		36			24
			808			687
Less accumulated depreciation			(179	)		(147	)
Net property and equipment		$	629		$	540

Available for Sale Securities (Tables)

3 Months Ended

Mar. 31, 2016

Investments Debt And Equity Securities [Abstract]

Summary of Available for Sale Securities at Fair Value

Available for sale securities at March 31, 2016 and December 31, 2015 consist of the following:

			Gross		Gross
			Unrealized		Unrealized
	Amortized Cost		Gains		Losses			Fair Value
	(in thousands)
March 31, 2016
Cash and cash equivalents	$	43,439	$	—	$	—		$	43,439
Available for sale securities:
Certificates of deposit	$	22,326	—		—			$	22,326
U.S. Government debt securities		128,548		14		(36	)		128,526
Corporate debt securities		22,739		8		(13	)		22,734
Total available for sale securities	$	173,613	$	22	$	(49	)	$	173,586
Total cash, cash equivalents, and available for sale securities	$	217,052	$	22	$	(49	)	$	217,025

			Gross		Gross
			Unrealized		Unrealized
	Amortized Cost		Gains		Losses			Fair Value
	(in thousands)
December 31, 2015
Cash and cash equivalents	$	49,778	$	—	$	—		$	49,778
Available for sale securities:
Certificates of deposit	$	21,505	—			—		$	21,505
U.S. Government debt securities		46,461	—			(185	)		46,276
Corporate debt securities		20,944		1		(50	)		20,895
Total available for sale securities	$	88,910	$	1	$	(235	)	$	88,676
Total cash, cash equivalents, and available for sale securities	$	138,688	$	1	$	(235	)	$	138,454

Estimated Fair Value of Available for Sale Securities at Fair Value by Contractual Maturity

The estimated fair value of the Company’s available for sale securities balance at March 31, 2016, by contractual maturity, is as follows:

Due in one year or less	$	122,978
Due after one year		50,608
Total available for sale securities	$	173,586

- Definition

Tabular disclosure of available-for-sale securities which includes, but is not limited to, changes in the cost basis and fair value, fair value and gross unrealized gain (loss), fair values by type of security, contractual maturity and classification, amortized cost basis, contracts to acquire securities to be accounted for as available-for-sale, debt maturities, transfers to trading, change in net unrealized holding gain (loss) net of tax, continuous unrealized loss position fair value, aggregate losses qualitative disclosures, other than temporary impairment (OTTI) losses or other disclosures related to available for sale securities.

+ References + Details

Fair Value of Financial Instruments (Tables)

3 Months Ended

Mar. 31, 2016

Fair Value Disclosures [Abstract]

Assets Measured or Disclosed at Fair Value on Recurring Basis

Assets measured or disclosed at fair value on a recurring basis as of March 31, 2016 are summarized below:

	Fair Value Measurements Using
	Level 1		Level 2		Level 3		Total
	(in thousands)
Assets:
Cash and cash equivalents	$	43,439	$	—	$	—	$	43,439
Certificates of deposit		—		22,326		—		22,326
U.S. Government debt securities		—		128,526		—		128,526
Corporate debt securities		—		22,734		—		22,734
	$	43,439	$	173,586	$	—	$	217,025

Assets measured or disclosed at fair value on a recurring basis as of December 31, 2015 are summarized below:

	Fair Value Measurements Using
	Level 1		Level 2		Level 3		Total
	(in thousands)
Assets:
Cash and cash equivalents	$	49,778	$	—	$	—	$	49,778
Certificates of deposit		—		21,505		—		21,505
U.S. Government debt securities		—		46,276		—		46,276
Corporate debt securities		—		20,895		—		20,895
	$	49,778	$	88,676	$	—	$	138,454

Accrued Expenses (Tables)

3 Months Ended

Mar. 31, 2016

Payables And Accruals [Abstract]

Components of Accrued Expenses

Accrued expenses are as follows:


	March 31, 2016		December 31, 2015
	(in thousands)
Accrued clinical	$	6,998	$	4,536
Professional fees		1,043		647
Accrued bonus		679		2,178
Accrued vacation		425		310
Accrued severance		319	—
Accrued payroll		218		518
Other		1,312		1,366
Total accrued expenses	$	10,994	$	9,555

Stockholders' Equity (Tables)

3 Months Ended

Mar. 31, 2016

Equity [Abstract]

Summary of Reserved Shares of Common Stock for Future Issuance

The Company has reserved for future issuance the following number of shares of common stock:


	March 31, 2016		December 31, 2015
Unvested restricted common stock		189,273		216,716
Common stock options and RSU's outstanding		3,112,081		2,231,057
Shares available for issuance under the 2014 Plan ⁽¹⁾		1,390,718		1,318,732
Shares available for issuance under the ESPP ⁽²⁾		819,734		440,304
Total		5,511,806		4,206,809

⁽¹⁾

On January 1, 2016, the shares reserved for future grants under the 2014 Plan increased by 986,800 shares pursuant to the 2014 Plan Evergreen Provision.

⁽²⁾

On February 28, 2016, the shares reserved for future issuance under the ESPP increased by 379,430 shares pursuant to the ESPP Evergreen Provision.

Compensation Cost Related to Share-Based Awards

The Company has recognized the following compensation cost related to share-based awards:

	Three months ended
	March 31, 2016		March 31, 2015
	(in thousands)
Research and development	$	392	$	565
General and administrative		859		563
Total	$	1,251	$	1,128

Schedule of Compensation Expense by Type of Award

Compensation expense by type of award:

	Three months ended
	March 31, 2016			March 31, 2015
	(in thousands)
Stock options	$	1,128		$	699
Restricted stock		(14	)		400
Restricted stock units		106			8
Employee stock purchase plan		31			21
Total	$	1,251		$	1,128

Commitments and Contingencies (Tables)

3 Months Ended

Mar. 31, 2016

Commitments And Contingencies Disclosure [Abstract]

Schedule of Future Minimum Payments Required Under Leases

At March 31, 2016, the Company’s future minimum payments required under these leases are as follows:

	Operating		Capital
	Lease		Lease			Total
	(in thousands)
2016	$	2,140	$	9		$	2,149
2017		3,121		9			3,130
2018		3,121		5			3,126
2019		3,121		—			3,121
2020		3,121		—			3,121
Thereafter		17,685		—			17,685
Total	$	32,309		23		$	32,332
Less amount representing interest				(1	)
Present value of minimum lease payments at March 31, 2016			$	22

- Definition

Tabular disclosure of a lessee's leasing arrangements including: (1) the basis on which contingent rental payments are determined, (2) the existence and terms of renewal or purchase options and escalation clauses, (3) restrictions imposed by lease arrangements, such as those concerning dividends, additional debt, and further leasing, (4) rent holidays, rent concessions, or leasehold improvement incentives and unusual provisions or conditions. Disclosure may also include the specific period used to amortize material leasehold improvements made at the inception of the lease or during the lease term. Additionally, for operating leases having initial or remaining noncancelable lease terms in excess of one year: (a) future minimum rental payments required as of the date of the latest balance sheet presented, in the aggregate and for each of the five succeeding fiscal years, (b) the total of minimum rentals to be received in the future under noncancelable subleases as of the date of the latest balance sheet presented, and (c) for all operating leases, rental expense for each period for which an income statement is presented, with separate amounts for minimum rentals, contingent rentals, and sublease rentals. Rental payments under leases with terms of a month or less that were not renewed need not be included.

+ References + Details

Net Loss per Share (Tables)

3 Months Ended

Mar. 31, 2016

Earnings Per Share [Abstract]

Anti-Dilutive Securities Excluded from Calculation of Diluted Net Loss per Share

The shares in the table below were excluded from the calculation of diluted net loss per share, prior to the use of the treasury stock method, due to their anti-dilutive effect:

	Three months ended
	March 31, 2016		March 31, 2015
Outstanding stock options		2,706,718		1,935,099
Unvested restricted stock		189,273		345,019
Unvested restricted stock units		405,363		27,875
Total		3,301,354		2,307,993

Nature of Organization and Operations - Additional Information (Details)		1 Months Ended		3 Months Ended	15 Months Ended
	Dec. 11, 2015 USD ($)	Jan. 31, 2016 USD ($) $ / shares shares	Dec. 31, 2015 USD ($)	Mar. 31, 2016 USD ($) Patient	Mar. 31, 2016 USD ($) Underwritten_offering
Nature Of Organization And Operations [Line Items]
Cash and cash equivalents and available for sale securities			$ 138,454,000	$ 217,025,000	$ 217,025,000
IPO
Nature Of Organization And Operations [Line Items]
Proceeds from issuance of initial public offering					$ 187,400,000
Number of underwritten public offerings \| Underwritten_offering					3
Follow-on public offering
Nature Of Organization And Operations [Line Items]
Proceeds from issuance of initial public offering		$ 61,000,000
Sale of stock, price per share \| $ / shares		$ 9.00
Follow-on public offering \| Common Stock
Nature Of Organization And Operations [Line Items]
Issuance of common stock, shares \| shares		7,250,000
Phase 3 vadadustat program \| Minimum
Nature Of Organization And Operations [Line Items]
Cost of program per patient				80,000
Phase 3 vadadustat program \| Maximum
Nature Of Organization And Operations [Line Items]
Cost of program per patient				$ 85,000
PRO2TECT
Nature Of Organization And Operations [Line Items]
Number of patients \| Patient				3,100
INNO2VATE
Nature Of Organization And Operations [Line Items]
Number of patients \| Patient				2,600
Development and Commercialize Collaboration Agreement
Nature Of Organization And Operations [Line Items]
Collaborative arrangement right description				In December 2015, we entered into a collaboration agreement with Mitsubishi Tanabe to develop and commercialize vadadustat in Japan and certain other countries in Asia for total milestone payments of up to $350.0 million, including up to $100.0 million in upfront and development payments, of which $40.0 million was received in January 2016. If Japanese patients are not included in either the global Phase 3 PRO2TECT or INNO2VATE programs, $20.0 million of the $40.0 million received would be used to fund further local development of vadadustat in Japan or be refunded to Mitsubishi. In addition, we will receive tiered double-digit royalty payments on sales of vadadustat.
Development and Commercialize Collaboration Agreement \| Upfront
Nature Of Organization And Operations [Line Items]
Milestone revenue		$ 40,000,000
Milestone revenue refundable		$ 20,000,000
Development and Commercialize Collaboration Agreement \| Maximum
Nature Of Organization And Operations [Line Items]
Milestone revenue			350,000,000
Development and Commercialize Collaboration Agreement \| Maximum \| Upfront
Nature Of Organization And Operations [Line Items]
Milestone revenue	$ 100,000,000		$ 100,000,000

Summary of Significant Accounting Policies - Additional Information (Details)	3 Months Ended
	Mar. 31, 2016 USD ($) Segment	Mar. 31, 2015 USD ($)
Summary Of Significant Accounting Policies [Line Items]
Number of operating segments \| Segment	1
Significant uncertain tax positions	$ 0	$ 0
Revenue	$ 0
Expected dividend yield	0.00%
Depreciation expense	$ 32,000	$ 19,000
Minimum
Summary Of Significant Accounting Policies [Line Items]
Property, plant and equipment, Useful life	3 years
Maximum
Summary Of Significant Accounting Policies [Line Items]
Property, plant and equipment, Useful life	7 years

Summary of Significant Accounting Policies - Summary of Property and Equipment and Related Accumulated Depreciation (Details) - USD ($) $ in Thousands	3 Months Ended
	Mar. 31, 2016	Dec. 31, 2015
Property, Plant and Equipment [Line Items]
Gross property, plant and equipment	$ 808	$ 687
Less accumulated depreciation	(179)	(147)
Net property and equipment	$ 629	540
Maximum
Property, Plant and Equipment [Line Items]
Property, plant and equipment, Useful life	7 years
Computer equipment and software
Property, Plant and Equipment [Line Items]
Property, plant and equipment, Useful life	3 years
Gross property, plant and equipment	$ 397	300
Furniture and fixtures
Property, Plant and Equipment [Line Items]
Property, plant and equipment, Useful life	5 years
Gross property, plant and equipment	$ 244	243
Equipment
Property, Plant and Equipment [Line Items]
Property, plant and equipment, Useful life	7 years
Gross property, plant and equipment	$ 50	50
Leasehold improvements
Property, Plant and Equipment [Line Items]
Leasehold improvements, Useful Life	Shorter of the useful life or remaining lease term (3 years)
Gross property, plant and equipment	$ 81	70
Leasehold improvements \| Maximum
Property, Plant and Equipment [Line Items]
Property, plant and equipment, Useful life	3 years
Office equipment under capital lease
Property, Plant and Equipment [Line Items]
Property, plant and equipment, Useful life	3 years
Gross property, plant and equipment	$ 36	$ 24

Available for Sale Securities - Summary of Available for Sale Securities at Fair Value (Details) - USD ($) $ in Thousands	Mar. 31, 2016	Dec. 31, 2015	Mar. 31, 2015	Dec. 31, 2014
Schedule Of Available For Sale Securities [Line Items]
Amortized Cost	$ 173,613	$ 88,910
Gross Unrealized Gains	22	1
Gross Unrealized Losses	(49)	(235)
Fair Value	173,586	88,676
Cash and cash equivalents	43,439	49,778	$ 27,887	$ 32,780
Cash and cash equivalents, fair value	43,439	49,778
Total cash, cash equivalents, and available for sale securities - Amortized Cost	217,052	138,688
Cash and cash equivalents and available for sale securities	217,025	138,454
U.S. Government debt securities
Schedule Of Available For Sale Securities [Line Items]
Amortized Cost	128,548	46,461
Gross Unrealized Gains	14
Gross Unrealized Losses	(36)	(185)
Fair Value	128,526	46,276
Certificates of deposit
Schedule Of Available For Sale Securities [Line Items]
Amortized Cost	22,326	21,505
Fair Value	22,326	21,505
Corporate debt securities
Schedule Of Available For Sale Securities [Line Items]
Amortized Cost	22,739	20,944
Gross Unrealized Gains	8	1
Gross Unrealized Losses	(13)	(50)
Fair Value	$ 22,734	$ 20,895

Available for Sale Securities - Estimated Fair Value of Available for Sale Securities by Contractual Maturity (Details) $ in Thousands	Mar. 31, 2016 USD ($)
Investments Debt And Equity Securities [Abstract]
Due in one year or less	$ 122,978
Due after one year	50,608
Total available for sale securities	$ 173,586

Fair Value of Financial Instruments - Assets Measured or Disclosed at Fair Value on Recurring Basis (Detail) - USD ($)	Mar. 31, 2016	Dec. 31, 2015
Level 3
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Assets Measured at fair value on recurring basis	$ 0	$ 0
Fair value measurements recurring
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Assets Measured at fair value on recurring basis	217,025,000	138,454,000
Fair value measurements recurring \| U.S. Government debt securities
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Assets Measured at fair value on recurring basis	128,526,000	46,276,000
Fair value measurements recurring \| Level 1
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Assets Measured at fair value on recurring basis	43,439,000	49,778,000
Fair value measurements recurring \| Level 2
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Assets Measured at fair value on recurring basis	173,586,000	88,676,000
Fair value measurements recurring \| Level 2 \| U.S. Government debt securities
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Assets Measured at fair value on recurring basis	128,526,000	46,276,000
Fair value measurements recurring \| Cash and cash equivalents
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Assets Measured at fair value on recurring basis	43,439,000	49,778,000
Fair value measurements recurring \| Cash and cash equivalents \| Level 1
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Assets Measured at fair value on recurring basis	43,439,000	49,778,000
Fair value measurements recurring \| Certificates of deposit
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Assets Measured at fair value on recurring basis	22,326,000	21,505,000
Fair value measurements recurring \| Certificates of deposit \| Level 2
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Assets Measured at fair value on recurring basis	22,326,000	21,505,000
Fair value measurements recurring \| Corporate debt securities
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Assets Measured at fair value on recurring basis	22,734,000	20,895,000
Fair value measurements recurring \| Corporate debt securities \| Level 2
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Assets Measured at fair value on recurring basis	$ 22,734,000	$ 20,895,000

Fair Value of Financial Instruments - Additional Information (Detail) - Level 3 - USD ($)	Mar. 31, 2016	Dec. 31, 2015
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Assets Measured at fair value on recurring basis	$ 0	$ 0
Liabilities measured at fair value on recurring basis	$ 0	$ 0

Accrued Expenses - Components of Accrued Expenses (Details) - USD ($) $ in Thousands	Mar. 31, 2016	Dec. 31, 2015
Payables And Accruals [Abstract]
Accrued clinical	$ 6,998	$ 4,536
Professional fees	1,043	647
Accrued bonus	679	2,178
Accrued vacation	425	310
Accrued severance	319
Accrued payroll	218	518
Other	1,312	1,366
Total accrued expenses	$ 10,994	$ 9,555

Accrued Expenses - Additional Information (Details) $ in Millions	1 Months Ended	3 Months Ended
	Jan. 31, 2016 Employee	Mar. 31, 2016 USD ($)	Mar. 31, 2015 USD ($)
Separation Agreement 1
Accrued Expenses [Line Items]
Severance agreement description		In August 2014, the Company entered into a separation agreement with an employee. The Company recorded the expense and liability associated with the separation agreement ratably over the period from August 5, 2014 through December 31, 2015 because the severance payments were subject to continued service and forfeiture until December 31, 2015. During the first quarter of 2015, the Company recorded severance expense in the amount of approximately $0.1 million, which was recorded to research and development expense. Payments under this separation agreement were paid out through December 2015.
Severance expense			$ 0.1
Separation Agreement 1 \| Paid Out Through December 2015
Accrued Expenses [Line Items]
Payment date for severance costs		were paid out through December 2015
Separation Agreement 2
Accrued Expenses [Line Items]
Severance agreement description		In January 2016, the Company entered into separation agreements with two employees. During the first quarter of 2016, the Company recorded severance expense in the amount of $0.4 million, of which $0.2 million was recorded to general and administrative expense and $0.2 million was recorded to research and development expense. During the first quarter of 2016, approximately $0.1 million was paid out of the severance accrual.
Severance expense		$ 0.4
Payment out of severance accrual		0.1
Number of employees \| Employee	2
Separation Agreement 2 \| General and Administrative Expense
Accrued Expenses [Line Items]
Severance expense		0.2
Separation Agreement 2 \| Research and Development Expense
Accrued Expenses [Line Items]
Severance expense		$ 0.2

Stockholders' Equity - Additional Information (Details) - USD ($)			3 Months Ended
	Feb. 22, 2016	Dec. 23, 2013	Mar. 31, 2016	Mar. 31, 2015	Dec. 31, 2015	Dec. 31, 2014	Feb. 28, 2014
Class of Stock [Line Items]
Common stock, shares authorized			175,000,000		175,000,000	175,000,000
Common stock, par value			$ 0.00001		$ 0.00001	$ 0.00001
Common stock, shares issued			37,946,010		30,662,218
Common stock, shares outstanding			37,946,010		30,662,218
Preferred stock, shares authorized			25,000,000		25,000,000
Preferred stock, par value			$ 0.00001		$ 0.00001
Preferred stock, shares issued			0		0
Preferred stock, shares outstanding			0		0
Common stock, shares reserved for future issuance			5,511,806		4,206,809
Stock-based compensation expense (reversal)			$ 1,251,000	$ 1,128,000
Restricted Stock Units
Class of Stock [Line Items]
Vesting periods			3 years
Stock-based compensation expense (reversal)			$ 106,000	8,000
Total compensation cost to be recognized			$ 2,900,000
Restricted Stock Units \| Share Based Compensation Award Tranche One
Class of Stock [Line Items]
Vesting percentage			100.00%
Stock Options
Class of Stock [Line Items]
Shares issued	624,275
Stock-based compensation description			Options vest in installments of (i) 25% at the one year anniversary and (ii) in either 36 or 48 equal monthly or 12 equal quarterly installments beginning in the thirteenth month after the initial Vesting Commencement Date (as defined) or grant date, subject to the employee’s continuous service with the Company
Options expiration after the date of grant			10 years
Stock-based compensation expense (reversal)			$ 1,128,000	699,000
Stock Options \| Share Based Compensation Award Tranche One
Class of Stock [Line Items]
Vesting percentage			25.00%
Restricted Stock
Class of Stock [Line Items]
Common stock, shares reserved for future issuance			189,273		216,716
Vesting periods		1 year
Stock-based compensation expense (reversal)			$ (14,000)	$ 400,000
Granted, Weighted-Average Grant Date Fair Value		$ 7.42
Restricted stock requisite service period description			The awards of restricted stock contained a performance condition wherein vesting is contingent upon the Company’s consummation of a liquidity event, as defined, prior to the fifth anniversary of the date of grant. Certain of the awards of restricted stock have a requisite service period that was complete upon grant. The remainder of the awards of restricted stock have a requisite service period of four years whereby the award vests 25% on the one year anniversary of the Vesting Commencement Date (as defined), then ratably on the first day of each calendar quarter for 12 quarters, subject to continuous service by the individual and achievement of the performance target
Grant-date fair value of restricted stock issued		$ 3,900,000
Award requisite service period		4 years
Restricted Stock \| Share Based Compensation Award Tranche One
Class of Stock [Line Items]
Vesting percentage		25.00%
Restricted Stock \| Employees
Class of Stock [Line Items]
Granted, Shares		450,224
Restricted Stock \| Non Employees
Class of Stock [Line Items]
Granted, Shares		79,067
Stock-based compensation expense (reversal)			$ (14,000)
Maximum \| Stock Options
Class of Stock [Line Items]
Vesting periods			48 months
Minimum \| Stock Options
Class of Stock [Line Items]
Vesting periods			12 months
2014 Plans
Class of Stock [Line Items]
Common stock, shares reserved for future issuance			1,390,718		1,318,732		1,785,000
Incremental rate at which the shares reserved for issuance increase			3.00%
2014 Plans \| Employees
Class of Stock [Line Items]
Granted, Shares			693,775
2014 Plans \| Director
Class of Stock [Line Items]
Granted, Shares			0
2014 Plans \| Restricted Stock Units
Class of Stock [Line Items]
Granted, Shares			382,338
2014 Plans \| Maximum
Class of Stock [Line Items]
Common stock, shares reserved for future issuance			1,131,937
2014 Employee Stock Purchase Plan
Class of Stock [Line Items]
Maximum shares of common stock authorized to be issued			262,500
Percentage of increase on common stock authorized on each anniversary			1.00%
Number of common stock outstanding			739,611
Common stock shares authorized percentage under the plan			5.00%
Purchase price at the end of offering period			85.00%
Offering period			6 months
Shares issued			0
Stock-based compensation expense (reversal)			$ 31,000

Stockholders' Equity - Summary of Reserved Shares of Common Stock for Future Issuance (Details) - shares	Mar. 31, 2016	Dec. 31, 2015	Feb. 28, 2014
Class of Stock [Line Items]
Common stock, shares reserved for future issuance	5,511,806	4,206,809
Restricted Stock
Class of Stock [Line Items]
Common stock, shares reserved for future issuance	189,273	216,716
Stock Options and Restricted Stock Units
Class of Stock [Line Items]
Common stock, shares reserved for future issuance	3,112,081	2,231,057
Employee Stock Purchase Plan
Class of Stock [Line Items]
Common stock, shares reserved for future issuance	819,734	440,304
2014 Plans
Class of Stock [Line Items]
Common stock, shares reserved for future issuance	1,390,718	1,318,732	1,785,000

Stockholders' Equity - Summary of Reserved Shares of Common Stock for Future Issuance (Parenthetical) (Details) - shares	Feb. 28, 2016	Jan. 01, 2016
2014 Plans
Class of Stock [Line Items]
Increase in common stock, shares reserved for future issuance		986,800
Employee Stock Purchase Plan
Class of Stock [Line Items]
Increase in common stock, shares reserved for future issuance	379,430

Stockholders Equity - Compensation Cost Related to Share-Based Awards (Details) - USD ($) $ in Thousands	3 Months Ended
	Mar. 31, 2016	Mar. 31, 2015
Employee Service Share-based Compensation, Allocation of Recognized Period Costs [Line Items]
Stock-based compensation expense	$ 1,251	$ 1,128
Research and Development
Employee Service Share-based Compensation, Allocation of Recognized Period Costs [Line Items]
Stock-based compensation expense	392	565
General and Administrative
Employee Service Share-based Compensation, Allocation of Recognized Period Costs [Line Items]
Stock-based compensation expense	$ 859	$ 563

Stockholders Equity - Schedule of Compensation Expense by Type of Award (Details) - USD ($) $ in Thousands	3 Months Ended
	Mar. 31, 2016	Mar. 31, 2015
Employee Service Share-based Compensation, Allocation of Recognized Period Costs [Line Items]
Stock-based compensation expense	$ 1,251	$ 1,128
Stock Options
Employee Service Share-based Compensation, Allocation of Recognized Period Costs [Line Items]
Stock-based compensation expense	1,128	699
Restricted Stock
Employee Service Share-based Compensation, Allocation of Recognized Period Costs [Line Items]
Stock-based compensation expense	(14)	400
Restricted Stock Units
Employee Service Share-based Compensation, Allocation of Recognized Period Costs [Line Items]
Stock-based compensation expense	106	8
Employee Stock Purchase Plan
Employee Service Share-based Compensation, Allocation of Recognized Period Costs [Line Items]
Stock-based compensation expense	$ 31	$ 21

Income Taxes - Additional Information (Details) - USD ($)	3 Months Ended
	Mar. 31, 2016	Mar. 31, 2015
Income Tax Disclosure [Abstract]
Income tax provisions or benefits	$ 0	$ 0

Commitments and Contingencies - Additional Information (Details)	1 Months Ended			3 Months Ended		12 Months Ended
	Nov. 30, 2015 USD ($) ft²	Dec. 31, 2014 USD ($) ft²	Dec. 31, 2013 USD ($) ft²	Mar. 31, 2016 USD ($) ft²	Mar. 31, 2015 USD ($)	Dec. 31, 2015 USD ($)
Commitments And Contingencies [Line Items]
Lease period			3 years
Area of property leased \| ft²		8,530	6,837
Monthly lease payments		$ 45,000	$ 31,000	$ 32,000
Rent abatement			$ 31,000
Lease commenced and rental payments period			2014-01
Cash-collateralized irrevocable standby letter of credit			$ 125,345
Lease space occupancy date		Mar. 13, 2015
Lease expiration date				Dec. 31, 2016
Landlord contribution for leasehold improvements		$ 100,000
Additional cash security deposit paid to landlord		$ 179,130
Rent expense associated with lease				$ 400,000	$ 100,000
Contract cost incurred in research and development activities				13,700,000		$ 5,200,000
Total remaining contract costs				$ 238,800,000
Office Equipment One
Commitments And Contingencies [Line Items]
Capital leases term				3 years
Capital lease commencement period				2014-02
Office Equipment Two
Commitments And Contingencies [Line Items]
Capital leases term				3 years
Capital lease commencement period				2015-04
Office Equipment Three
Commitments And Contingencies [Line Items]
Capital leases term				3 years
Capital lease commencement period				2016-02
Second Amendment
Commitments And Contingencies [Line Items]
Lease period				10 years
Area of property leased \| ft²	23,189
Monthly lease payments	$ 107,000
Lease expiration date				Aug. 31, 2026
Additional cash security deposit paid to landlord	$ 976,382
Square feet of leased property \| ft²	16,222
Operating lease payments commencement date				May 01, 2017
Second Amendment \| Other Assets
Commitments And Contingencies [Line Items]
Additional cash security deposit paid to landlord				$ 1,280,857		$ 304,475
Second Amendment \| Delivered on December 31, 2015
Commitments And Contingencies [Line Items]
Monthly lease payments				$ 20,000
Square feet of leased property \| ft²				3,066
Operating lease payments commencement date				Apr. 01, 2016
Second Amendment \| Delivered on April 11, 2016
Commitments And Contingencies [Line Items]
Monthly lease payments				$ 110,000
Square feet of leased property \| ft²				16,739
Operating lease payments commencement date				Sep. 11, 2016
Second Amendment \| Delivered on January 1, 2017
Commitments And Contingencies [Line Items]
Monthly lease payments				$ 23,000
Square feet of leased property \| ft²				3,384
Operating lease payments commencement date				May 01, 2017

Commitments and Contingencies - Schedule of Future Minimum Payments Required Under Leases (Details) $ in Thousands	Mar. 31, 2016 USD ($)
Commitments And Contingencies Disclosure [Abstract]
Operating Lease, 2016	$ 2,140
Operating Lease, 2017	3,121
Operating Lease, 2018	3,121
Operating Lease, 2019	3,121
Operating Lease, 2020	3,121
Operating Lease, Thereafter	17,685
Operating Lease, Total	32,309
Capital Lease, 2016	9
Capital Lease, 2017	9
Capital Lease, 2018	5
Capital Lease	23
Capital Lease, Less amount representing interest	(1)
Capital Lease, Present value of minimum lease payments at March 31, 2016	22
Operating and Capital Lease, 2016	2,149
Operating and Capital Lease, 2017	3,130
Operating and Capital Lease, 2018	3,126
Operating and Capital Lease, 2019	3,121
Operating and Capital Lease, 2020	3,121
Operating and Capital Lease, Thereafter	17,685
Operating and Capital Lease, Total	$ 32,332

Significant Agreements - Additional Information (Details) - USD ($)		1 Months Ended
	Dec. 11, 2015	Jan. 31, 2016	Dec. 31, 2015	Mar. 31, 2016
Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]
Deferred revenue, noncurrent				$ 40,000,000
Development and Commercialize Collaboration Agreement
Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]
Maximum percentage of royalty payments	20.00%
Collaboration Agreement, expiration period	10 years
Collaboration Agreement, notice period for termination	12 months
Deferred revenue, noncurrent				$ 40,000,000
Development and Commercialize Collaboration Agreement \| Upfront
Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]
Milestone revenue		$ 40,000,000
Milestone revenue refundable		$ 20,000,000
Development and Commercialize Collaboration Agreement \| Maximum
Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]
Milestone revenue			$ 350,000,000
Development and Commercialize Collaboration Agreement \| Maximum \| Upfront
Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]
Milestone revenue	$ 100,000,000		$ 100,000,000
Development and Commercialize Collaboration Agreement \| Maximum \| Additional Milestone Payments
Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]
Milestone revenue	$ 250,000,000

Employee Retirement Plan - Additional Information (Details) - USD ($)	3 Months Ended
	Mar. 31, 2016	Mar. 31, 2015
Compensation And Retirement Disclosure [Abstract]
Employers contributions to defined contribution plan	$ 100,000	$ 0
Employees eligibility age to participate in retirement plan	21 years

Net Loss per Share - Anti-Dilutive Securities Excluded from Calculation of Diluted Net Loss per Share (Details) - shares	3 Months Ended
	Mar. 31, 2016	Mar. 31, 2015
Antidilutive Securities Excluded from Computation of Earnings Per Share [Line Items]
Anti-dilutive securities excluded from calculation of diluted net loss per share	3,301,354	2,307,993
Options to purchase common stock
Antidilutive Securities Excluded from Computation of Earnings Per Share [Line Items]
Anti-dilutive securities excluded from calculation of diluted net loss per share	2,706,718	1,935,099
Restricted Stock
Antidilutive Securities Excluded from Computation of Earnings Per Share [Line Items]
Anti-dilutive securities excluded from calculation of diluted net loss per share	189,273	345,019
Restricted Stock Units
Antidilutive Securities Excluded from Computation of Earnings Per Share [Line Items]
Anti-dilutive securities excluded from calculation of diluted net loss per share	405,363	27,875