Dicerna Pharmaceuticals (DRNA) 10-K12 Mar 152014 FY Annual reportFinancial data

Company Profile

Document_and_Entity_Informatio

Document and Entity Information (USD $)	12 Months Ended
In Millions, except Share data, unless otherwise specified	Dec. 31, 2014	Mar. 11, 2015	Jun. 30, 2014
Document And Entity Information [Abstract]
Document Type	10-K
Amendment Flag	FALSE
Document Period End Date	31-Dec-14
Document Fiscal Year Focus	2014
Document Fiscal Period Focus	FY
Trading Symbol	DRNA
Entity Registrant Name	DICERNA PHARMACEUTICALS INC
Entity Central Index Key	1399529
Current Fiscal Year End Date	-19
Entity Well-known Seasoned Issuer	No
Entity Current Reporting Status	Yes
Entity Voluntary Filers	No
Entity Filer Category	Non-accelerated Filer
Entity Common Stock, Shares Outstanding		17,820,985
Entity Public Float			$118.80

Consolidated_Balance_Sheets

Consolidated Balance Sheets (USD $)	Dec. 31, 2014	Dec. 31, 2013
In Thousands, unless otherwise specified	Dec. 31, 2014	Dec. 31, 2013
CURRENT ASSETS:
Cash and cash equivalents	$26,067	$46,595
Held-to-maturity investments	70,055
Prepaid expenses and other current assets	1,194	2,058
Total current assets	97,316	48,653
NONCURRENT ASSETS:
Property and equipment-net	2,165	877
Held-to-maturity investments	2,501
Assets held in restriction	1,380	264
Other assets	243
Total noncurrent assets	6,289	1,141
TOTAL ASSETS	103,605	49,794
CURRENT LIABILITIES:
Accounts payable	1,237	1,700
Accrued expenses and other current liabilities	3,845	1,286
Deferred rent	77	105
Current portion of long-term debt		4,587
Total current liabilities	5,159	7,678
NONCURRENT LIABILITIES:
Security deposit	106
Long-term debt-net of current portion		260
Preferred stock warrant liability		529
Total noncurrent liabilities	106	789
TOTAL LIABILITIES	5,265	8,467
COMMITMENTS AND CONTINGENCIES (Note 13)
STOCKHOLDERS' EQUITY (DEFICIT):
Preferred stock, $0.0001 par value-5,000,000 shares and no shares authorized at December 31, 2014 and December 31, 2013, respectively; no shares issued and outstanding at December 31, 2014 and December 31, 2013, respectively
Common stock, $0.0001 par value-150,000,000 and 15,000,000 shares authorized; 17,786,867 and 38,226 shares issued and outstanding at December 31, 2014 and 2013, respectively	3	1
Additional paid-in capital	231,741	16,545
Accumulated deficit	-133,404	-85,465
Total stockholders' equity / (deficit)	98,340	-68,919
TOTAL LIABILITIES AND STOCKHOLDERS' EQUITY / (DEFICIT)	103,605	49,794
Series A Redeemable Convertible Preferred Stock [Member]
REDEEMABLE CONVERTIBLE PREFERRED STOCK, $0.0001 PAR VALUE-AUTHORIZED 11,070,000 SHARES:
REDEEMABLE CONVERTIBLE PREFERRED STOCK, Value		21,400
Series B Redeemable Convertible Preferred Stock [Member]
REDEEMABLE CONVERTIBLE PREFERRED STOCK, $0.0001 PAR VALUE-AUTHORIZED 11,070,000 SHARES:
REDEEMABLE CONVERTIBLE PREFERRED STOCK, Value		29,050
Series C Redeemable Convertible Preferred Stock [Member]
REDEEMABLE CONVERTIBLE PREFERRED STOCK, $0.0001 PAR VALUE-AUTHORIZED 11,070,000 SHARES:
REDEEMABLE CONVERTIBLE PREFERRED STOCK, Value		$59,796

Consolidated_Balance_Sheets_Pa

Consolidated Balance Sheets (Parenthetical) (USD $)	Dec. 31, 2014	Dec. 31, 2013
REDEEMABLE CONVERTIBLE PREFERRED STOCK, PAR VALUE	0.0001	$0.00
REDEEMABLE CONVERTIBLE PREFERRED STOCK, SHARES AUTHORIZED	11,070,000	11,070,000
REDEEMABLE CONVERTIBLE PREFERRED STOCK, SHARES ISSUED		10,589,434
Preferred stock, par value	0.0001	$0.00
Preferred stock, shares authorized	5,000,000	0
Preferred stock, shares issued	0	0
Preferred stock, shares outstanding	0	0
Common stock, par value	0.0001	$0.00
Common stock, shares authorized	150,000,000	15,000,000
Common stock, shares issued	17,786,867	38,226
Common stock, shares outstanding	17,786,867	38,226
Series A Redeemable Convertible Preferred Stock [Member]
REDEEMABLE CONVERTIBLE PREFERRED STOCK, SHARES AUTHORIZED	0	880,000
REDEEMABLE CONVERTIBLE PREFERRED STOCK, SHARES ISSUED	0	855,996
REDEEMABLE CONVERTIBLE PREFERRED STOCK, SHARES OUTSTANDING	0	855,996
REDEEMABLE CONVERTIBLE PREFERRED STOCK, Aggregate liquidation preference		$21,400,000
Series B Redeemable Convertible Preferred Stock [Member]
REDEEMABLE CONVERTIBLE PREFERRED STOCK, SHARES AUTHORIZED	0	1,190,000
REDEEMABLE CONVERTIBLE PREFERRED STOCK, SHARES ISSUED	0	1,162,021
REDEEMABLE CONVERTIBLE PREFERRED STOCK, SHARES OUTSTANDING	0	1,162,021
REDEEMABLE CONVERTIBLE PREFERRED STOCK, Aggregate liquidation preference		29,050,000
Series C Redeemable Convertible Preferred Stock [Member]
REDEEMABLE CONVERTIBLE PREFERRED STOCK, SHARES AUTHORIZED	0	9,000,000
REDEEMABLE CONVERTIBLE PREFERRED STOCK, SHARES ISSUED	0	8,571,417
REDEEMABLE CONVERTIBLE PREFERRED STOCK, SHARES OUTSTANDING	0	8,571,417
REDEEMABLE CONVERTIBLE PREFERRED STOCK, Aggregate liquidation preference	0	$60,000,000

Consolidated_Statements_of_Ope

Consolidated Statements of Operations (USD $)	12 Months Ended
In Thousands, except Share data, unless otherwise specified	Dec. 31, 2014	Dec. 31, 2013	Dec. 31, 2012
Income Statement [Abstract]
Revenue	$0	$0	$7,015
Operating expenses:
Research and development	29,453	11,558	11,565
General and administrative	15,648	5,820	4,700
Total operating expenses	45,101	17,378	16,265
Loss from operations	-45,101	-17,378	-9,250
Other income (expense):
Preferred stock warrant liability remeasurement	-2,559	126	469
Loss on extinguishment of debt	-143	-318
Interest income	63	4	2
Interest expense	-199	-952	-1,342
Total other income (expense)	-2,838	-1,140	-871
Net loss	-47,939	-18,518	-10,121
Less: Accretion and dividends on redeemable convertible preferred stock	204	2,388	4,097
Net loss attributable to common stockholders	($48,143)	($20,906)	($14,218)
Net loss per share attributable to common stockholders-basic and diluted	($3)	($709.57)	($516)
Weighted average shares outstanding-basic and diluted	16,070,054	29,463	27,554

Consolidated_Statements_of_Red

Consolidated Statements of Redeemable Convertible Preferred Stock and Stockholders' Equity / (Deficit) (USD $)	Total	Series A Redeemable Convertible Preferred Stock [Member]	Series B Redeemable Convertible Preferred Stock [Member]	Series C Redeemable Convertible Preferred Stock [Member]	Common Stock [Member]	Additional Paid in Capital [Member]	Accumulated Deficit [Member]
In Thousands, except Share data	Total	Series A Redeemable Convertible Preferred Stock [Member]	Series B Redeemable Convertible Preferred Stock [Member]	Series C Redeemable Convertible Preferred Stock [Member]	Common Stock [Member]	Additional Paid in Capital [Member]	Accumulated Deficit [Member]
Balance at beginning of period at Dec. 31, 2011	($50,633)				$1		($50,634)
Balance at beginning of period at Dec. 31, 2011		27,990	32,161
Balance at beginning of period (in shares) at Dec. 31, 2011					27,522
Balance at beginning of period (in shares) at Dec. 31, 2011		855,996	1,162,021
Accretion of preferred stock issuance costs	-61	26	35			-61
Accrued dividends	-4,036	1,712	2,324			-71	-3,965
Vesting of restricted common stock	2					2
Vesting of restricted common stock (in shares)					50
Stock-based compensation	123					123
Exercise of common stock options	7					7
Exercise of common stock options (in shares)					281
Net loss	-10,121						-10,121
Balance at end of period at Dec. 31, 2012	-64,719				1		-64,720
Balance at end of period at Dec. 31, 2012		29,728	34,520
Balance at end of period (in shares) at Dec. 31, 2012					27,853
Balance at end of period (in shares) at Dec. 31, 2012		855,996	1,162,021
Issuance of Common Stock from initial public offering, net of underwriting fees and issuance costs of $10,751				56,780
Issuance of Common Stock from initial public offering, net of underwriting fees and issuance costs, Shares				8,142,891
Accretion of preferred stock issuance costs	-121	47	58	16		-121
Conversion of preferred stock to common stock				3,000
Issuance of Series C preferred stock, in satisfaction of bridge loan (in shares)				428,526
Conversion of preferred stock to common stock, Shares	10,589,434
Accrued dividends	-2,267	962	1,305			-40	-2,227
Deemed contribution of preferred stockholders		-9,337	-6,833
Deemed contribution of preferred stockholders	16,170					16,170
Vesting of restricted common stock (in shares)					15
Repurchase of unvested restricted common stock	-5					-5
Stock-based compensation	495					495
Exercise of common stock options	46					46
Exercise of common stock options (in shares)					10,358
Net loss	-18,518						-18,518
Balance at end of period at Dec. 31, 2013	-68,919				1	16,545	-85,465
Balance at end of period at Dec. 31, 2013	110,450	21,400	29,050	59,796
Balance at end of period (in shares) at Dec. 31, 2013					38,226
Balance at end of period (in shares) at Dec. 31, 2013		855,996	1,162,021	8,571,417
Issuance of Common Stock from initial public offering, net of underwriting fees and issuance costs of $10,752	92,750				1	92,749
Issuance of Common Stock from initial public offering, net of underwriting fees and issuance costs, Shares					6,900,000
Net exercise of common stock warrant	0	0	0	0	0	0	0
Net exercise of common stock warrant, Shares					12,702
Reclassification of warrants to purchase shares of redeemable convertible preferred stock into a warrant to purchase common stock	3,088					3,088
Accretion of preferred stock issuance costs	-204			204		-204
Conversion of preferred stock to common stock		-21,400	-29,050	-60,000
Conversion of preferred stock to common stock	110,452				1	110,451
Conversion of preferred stock to common stock, Shares		855,996	1,162,021	8,571,417	10,589,434
Vesting of restricted common stock (in shares)					4,000
Stock-based compensation	8,237					8,237
Exercise of common stock options	824					824
Exercise of common stock options (in shares)	239,853				239,853
Sale of common stock related to employee stock purchase plan	51					51
Sale of common stock related to employee stock purchase plan (in shares)					2,652
Net loss	-47,939						-47,939
Balance at end of period at Dec. 31, 2014	$98,340				$3	$231,741	($133,404)
Balance at end of period (in shares) at Dec. 31, 2014					17,786,867
Balance at end of period (in shares) at Dec. 31, 2014		0	0	0

Consolidated_Statements_of_Red1

Consolidated Statements of Redeemable Convertible Preferred Stock and Stockholders' Equity / (Deficit) (Parenthetical) (Series C Redeemable Convertible Preferred Stock [Member], USD $)	12 Months Ended
In Thousands, unless otherwise specified	Dec. 31, 2014	Dec. 31, 2013
Series C Redeemable Convertible Preferred Stock [Member]
Issuance of Series C preferred stock, issuance costs		$220
Underwriting fees and issuance costs	$10,751

Consolidated_Statements_of_Cas

Consolidated Statements of Cash Flows (USD $)	12 Months Ended
In Thousands, unless otherwise specified	Dec. 31, 2014	Dec. 31, 2013	Dec. 31, 2012
CASH FLOWS FROM OPERATING ACTIVITIES:
Net loss	($47,939)	($18,518)	($10,121)
Adjustments to reconcile net loss to net cash used in operating activities:
Deferred revenue			-1,014
Depreciation and amortization	848	740	776
Stock-based compensation	8,237	495	125
Loss on extinguishment of debt	143	318
Increase (Decrease) in fair value of preferred stock warrant	2,559	-126	-469
Changes in operating assets and liabilities:
Research and license receivable		5,018	-4,857
Prepaid expenses and other assets	-1,171	57	28
Accounts payable	-97	116	-216
Accrued expenses and other liabilities	2,684	1,016	49
Deferred rent	-28	-60	-38
Net cash used in operating activities	-34,764	-10,944	-15,737
CASH FLOWS FROM INVESTING ACTIVITIES-
Changes in assets held in restriction	-1,116
Purchases of property and equipment	-2,013	-413	-120
Maturities of held-to-maturity investments	9,995
Purchases of held-to-maturity investments	-82,627
Net cash used in investing activities	-75,761	-413	-120
CASH FLOWS FROM FINANCING ACTIVITIES:
Proceeds from issuance of common stock	875	46	7
Net proceeds from initial public offering	94,148
Repurchase of restricted common stock		-5
Payments of deferred issuance costs		-1,399
Proceeds from issuance of redeemable convertible preferred stock		57,000
Redeemable preferred stock issuance costs		-220
Proceeds from bridge loan financing		3,000
Payments of long-term debt fees			-136
Repayments of long-term debt principal	-5,026	-4,140	-2,834
Net cash provided by (used in) financing activities	89,997	54,282	-2,963
(DECREASE) INCREASE IN CASH AND CASH EQUIVALENTS	-20,528	42,925	-18,820
CASH AND CASH EQUIVALENTS-Beginning of year	46,595	3,670	22,490
CASH AND CASH EQUIVALENTS-End of year	26,067	46,595	3,670
NONCASH FINANCING ACTIVITIES:
Accretion of redeemable convertible preferred stock	204	2,388	4,097
Deemed contribution of preferred stockholders		16,170
Conversion of bridge loan financing		3,000
Tenant allowances		104
SUPPLEMENTAL CASH FLOW INFORMATION-
Warrant conversion to common stock	3,088
Cash paid for interest	$194	$771	$1,123

Nature_of_Business_and_Basis_o

Nature of Business and Basis of Presentation	12 Months Ended
Nature of Business and Basis of Presentation	Dec. 31, 2014
Accounting Policies [Abstract]
Nature of Business and Basis of Presentation	1. Nature of Business and Basis of Presentation

	Nature of business

	Dicerna Pharmaceuticals, Inc., and its subsidiaries, (the Company) is a biopharmaceutical company focused on the discovery and development of innovative treatments for rare inherited diseases involving the liver and for cancers that are genetically defined. The Company is using its proprietary RNA interference, (RNAi), technology platform, which the Company believes improves on existing RNAi technologies, to build a broad pipeline in these therapeutic areas. The Company intends to discover, develop and commercialize novel therapeutics either on its own or in collaboration with pharmaceutical partners.

	The Company continues to be subject to a number of risks common to companies in similar stages of development. Principal among these risks are the uncertainties of technological innovations, which are particularly high in the field of drug discovery and development, dependence on key individuals, development of the same or similar technological innovations by the Company’s competitors and protection of proprietary technology. The Company’s ability to fund its planned preclinical and clinical operations, including completion of its planned trials, is expected to depend on the amount and timing of cash receipts under its existing collaboration agreement, as well as any future collaboration or product sales and financing transactions.

	In February 2014, the Company completed the sale of 6,900,000 shares of common stock in its initial public offering (IPO) at a price to the public of $15.00 per share, resulting in net proceeds to the Company of $92,750 after deducting underwriting discounts and commissions of $7,245 and offering expenses of $3,506. The Company’s common stock began trading on the NASDAQ Global Market under the symbol “DRNA” on January 31, 2014. In connection with the close of the IPO, all of the outstanding shares of Series A mandatorily redeemable, convertible preferred stock (Series A preferred stock), Series B mandatorily redeemable, convertible preferred stock (Series B preferred stock) and Series C mandatorily redeemable, convertible preferred stock (Series C preferred stock) were converted into shares of common stock on a one-for-one basis immediately prior to the closing of the IPO. Following these transactions, the Company had a total of 17,786,867 shares of common stock issued and outstanding as of December 31, 2014. The significant increase in the shares outstanding in 2014 has impacted the year over year comparability of the Company’s net loss per share calculations.

Summary_of_Significant_Account

Summary of Significant Accounting Policies	12 Months Ended
	Dec. 31, 2014
Accounting Policies [Abstract]
Summary of Significant Accounting Policies	2. Summary of Significant Accounting Policies

	Basis of presentation and consolidation

	The accompanying consolidated financial statements have been prepared under accounting principles generally accepted in the United States of America, or GAAP, and include the accounts of the Company and its wholly-owned subsidiary. All intercompany balances and transactions have been eliminated in consolidation.

	Significant judgments and estimates

	The preparation of these financial statements is in conformity with GAAP which requires management to make estimates and assumptions that affect the reported amounts of assets and liabilities, and the disclosure of contingent assets and liabilities at the date of the consolidated financial statements, as well as the revenues and expenses incurred during the reporting periods. On an ongoing basis, the Company evaluate judgments and estimates, including those related to accrued expenses, revenue recognition, deferred revenue and stock-based compensation. The Company base estimates on historical experience and on various other factors that the Company believes are reasonable under the circumstances, the results of which form the basis for making judgments about the carrying value of assets and liabilities that are not apparent from other sources. Changes in estimates are reflected in reported results for the period in which they become known. Actual results could differ materially from those estimates.



	Cash equivalents

	Cash equivalents include all highly liquid investments maturing within 90 days from the date of purchase. Cash equivalents consist of money market funds as of December 31, 2014 and 2013 and are valued at cost, plus accrued interest, which approximates fair value.

	Held-to-Maturity Investments

	The Company accounts for its investments in marketable securities in accordance with FASB ASC 320, Investments—Debt and Equity Securities. The Company determines the appropriate classification of investments at the time of purchase and re-evaluates such designation as of each balance sheet date. Debt securities carried at amortized cost are classified as held-to-maturity when the Company has the positive intent and ability to hold the securities to maturity. At December 31, 2014, all of the Company’s investments are classified as held-to-maturity.

	Assets held in restriction

	As of December 31, 2014, assets held in restriction are comprised of two certificates of deposit that mature annually, and secure the Company’s outstanding letters of credit of $1,116 and $264 for the operating leases for office and laboratory space. The letters of credit are required to be maintained throughout the term of the Company’s leases which expire on December 1, 2020 and November 30, 2016, respectively. As of December 31, 2013, assets held in restriction is comprised of one certificate of deposit securing the Company’s outstanding letter of credit of $264, as noted above.

	Concentrations of credit risk

	Financial instruments that subject the Company to significant concentrations of credit risk consist of cash and cash equivalents, assets held in restriction and held-to-maturity investments. All of the Company’s cash and cash equivalents, assets held in restriction and held-to-maturity investments are invested in money market funds or U.S. Treasury or agency securities that management believes to be of high credit quality. During 2012, one counterparty accounted for all of the Company’s revenue.

	Deferred stock issuance costs

	Deferred stock issuance costs, which consisted of direct incremental legal and professional accounting fees relating to the Company’s IPO, totaling $1,784, were initially capitalized in 2013 and subsequently offset against IPO proceeds in 2014, when the offering became effective. No amounts were deferred as of December 31, 2014.

	Property and equipment

	Property and equipment are stated at cost. Major betterments are capitalized whereas expenditures for maintenance and repairs which do not improve or extend the life of the respective assets are charged to operations as incurred. Depreciation is provided using the straight-line method over the estimated useful lives:



	ASSET CATEGORY	USEFUL LIVES
	Office and computer equipment	3-5 years
	Laboratory equipment	5 years
	Furniture and fixtures	5 years
	Leasehold improvements	5 years or the remaining

		term of lease, if shorter

	Impairment of long-lived assets

	Long-lived assets are reviewed for impairment whenever events or changes in circumstances indicate that the carrying amount of the asset may not be recoverable. When such events occur, the Company compares the carrying amounts of the assets to their undiscounted expected future cash flows. If this comparison indicates that there is an impairment, the amount of the impairment is calculated as the difference between the carrying value and fair value of the related asset. For the years ended December 31, 2014, 2013 and 2012, no impairments have been recorded.

	Fair value of financial instruments

	The carrying amounts of the Company’s financial instruments include cash equivalents, accounts payable, and other accrued expenses, approximate their fair values due to their short duration. Management believes that the Company’s long-term debt bears interest at the prevailing market rate for instruments with similar characteristics and, accordingly, the carrying value of long-term debt also approximates their fair value.

	Segment and geographic information

	Operating segments are defined as components (business activity from which it earns revenue and incurs expenses) of an enterprise about which discrete financial information is available and regularly reviewed by the chief operating decision maker in deciding how to allocate resources and in assessing performance. The Company, through its Chief Executive Officer in his role as chief operating decision maker, views its operations and manages its business as one operating segment. All material long-lived assets of the Company are located in the United States.

	Revenue recognition

	The Company generates revenue from research collaboration and license agreements with third parties which contain multiple deliverables. The deliverables in the agreements include (a) the use of the Company’s technology and (b) research and development of product candidates. Such agreements may provide for consideration to the Company in the form of up-front payments, research and development services, milestone payments and royalties. Revenue is recognized when the following criteria have been met: (1) pervasive evidence of an arrangement exists; (2) delivery has occurred or services have been rendered and risk of loss has passed; (3) the seller’s price to the buyer is fixed or determinable; and (4) collectability is reasonably assured. Multiple-deliverable arrangements are analyzed to determine whether the deliverables can be separated or whether they must be accounted for as a single unit of accounting. When deliverables are separable, consideration received is allocated to the separate units of accounting based on the relative selling price method and the appropriate revenue recognition principles are applied to each unit.

	At the inception of each arrangement that includes payments for optional research or milestones, the Company evaluates whether each option or milestone is substantive and at risk to both parties on the basis of the contingent nature of the option or milestone. This evaluation includes an assessment of whether (1) the consideration is commensurate with either the entity’s performance to achieve the milestone or the enhancement of the value of the delivered item(s); (2) as a result of a specific outcome resulting from the entity’s performance to achieve the milestone; (3) the consideration relates solely to past performance; and (4) the consideration is reasonable relative to all of the deliverables and payment terms within the arrangement. Substantive options and milestones are recognized as revenue upon the achievement of the milestone, assuming all other revenue recognition criteria are met.

When the Company determines that an arrangement should be accounted for as a single unit of accounting, it must determine the period over which the performance obligations will be performed and revenue will be recognized. The Company’s only revenue to date results from a research collaboration and license agreement entered into in December 2009. Non-refundable up-front license fees under the agreement were initially recorded as deferred revenue upon receipt and are being recognized as revenue over the Company’s performance period as defined in the agreement. Research and development service revenue is recognized over the research term as the research and development services are provided. The cost of such services is reflected in research and development costs in the period in which it is incurred.



Royalty payments are recognized as revenue based on contract terms and reported sales of licensed products, when reported sales are reliably measurable and collectability is reasonably assured.

Research and development costs

Research and development costs consist of expenses incurred in performing research and development activities, including compensation and benefits for full-time research and development employees, an allocation of facility expenses, overhead expenses and other outside expenses. Research and development costs are expensed as incurred. Research and development costs that are paid in advance of performance are deferred as a prepaid expense and amortized over the service period as the services are provided.

Preferred stock warrant liability

Freestanding warrants related to shares that are redeemable, contingently redeemable, or for purchases of preferred stock that are not indexed to the Company’s own stock are classified as a liability on the Company’s balance sheet. The preferred stock warrants were recorded at fair value, estimated using the Black-Scholes option-pricing model for the year ended December 31, 2013 and until the conversion date of February 4, 2014, and marked to market at each balance sheet date with changes in the fair value of the liability recorded in the statements of operations. After the closing of the IPO, the preferred stock warrants were no longer classified as a liability subject to remeasurement as the preferred stock warrants became warrants to purchase shares of common stock. The Company classified the liability as noncurrent at December 31, 2013. There were no preferred stock warrants outstanding as of December 31, 2014.

Deferred Rent

Deferred rent consists of rent escalation payment terms, tenant improvement allowances and other incentives received from landlords related to the Company’s operating leases. Rent escalation represents the difference between actual operating lease payments due and straight-line rent expense, which is recorded by the Company over the term of the lease. Tenant improvement allowances and other incentives are recorded as deferred rent and amortized as a reduction of periodic rent expense, over the term of the applicable lease.

Redeemable convertible preferred stock

The Company initially records preferred stock that may be redeemed at the option of the holder or based on the occurrence of events not under the Company’s control outside of stockholders’ equity (deficit) at the value of the proceeds received or fair value, if lower, net of issuance costs. Subsequently, if it is probable that the preferred stock will become redeemable, the Company adjusts the carrying value to the redemption value over the period from the issuance date to the earliest possible redemption date using the effective interest method. If it is not probable that the preferred stock will become redeemable, the Company does not adjust the carrying value. In the absence of retained earnings these accretion charges are recorded against additional paid-in-capital, if any, and then to accumulated deficit.

Common stock valuation

Due to the absence of an active market for the Company’s common stock prior to the IPO, the Company utilized methodologies in accordance with the framework of the American Institute of Certified Public Accountants Technical Practice Aid, Valuation of Privately-Held Company Equity Securities Issued as Compensation , to estimate the fair value of its common stock. In determining the exercise prices for options granted, the Company has considered the fair value of the common stock as of the measurement date. The fair value of the common stock has been determined at each award grant date based upon a variety of factors, including the illiquid nature of the common stock, arm’s-length sales of the Company’s capital stock (including redeemable convertible preferred stock), the effect of the rights and preferences of the preferred stockholders, and the prospects of a liquidity event. Among other factors are the Company’s financial position and historical financial performance, the status of technological developments within the Company’s research, the composition and ability of the current research and management team, an evaluation or benchmark of the Company’s competition, and the current business climate in the marketplace. Significant changes to the key assumptions underlying the factors used could result in different fair values of common stock at each valuation date.

Stock-based compensation

The Company accounts for stock options granted as share-based payments at fair value, which is measured using the Black-Scholes option pricing model. The fair value measurement date for employee awards is the date of grant. The fair value measurement date for nonemployee awards is generally the date the performance of services is completed. Share-based compensation costs are recognized as expense over the requisite service period, which is generally the vesting period, on a straight-line basis for all time-vested awards.

For performance-based stock awards, compensation costs are first recorded when the Company determines that the achievement of such performance conditions is deemed probable. This determination requires significant judgment by management. At the probable date, the Company records a cumulative expense catch-up, with the remaining compensation cost being amortized over the remaining vesting period.

The Company accounts for restricted stock awards granted to employees at fair value, which is measured based upon the quoted closing market price per share on the date of grant, adjusted for assumed forfeitures. The compensation costs are recognized over the vesting period, commencing when the Company determines that it is probable that the awards will vest.

Share-based payments to nonemployees are remeasured at each reporting date and recognized as services are rendered, generally on a straight-line basis. The Company believes that the fair value of these awards is more reliably measurable than the fair value of the services rendered.

Income taxes

The Company records deferred tax assets and liabilities for the expected future tax consequences of temporary differences between the Company’s financial statement carrying amounts and the tax basis of assets and liabilities using enacted tax rates expected to be in effect in the years in which the differences are expected to reverse. A valuation allowance is provided to reduce the net deferred tax assets to the amount that will more likely than not be realized.

The Company also assesses the probability that the positions taken or expected to be taken in its income tax returns will be sustained by taxing authorities. A “more likely than not” (more than 50 percent) recognition threshold must be met before a tax benefit can be recognized. Tax positions that are more likely than not to be sustained are reflected in the Company’s financial statements. Tax positions are measured as the largest amount of tax benefit that is greater than 50 percent likely of being realized upon settlement with a taxing authority that has full knowledge of all relevant information. The difference between the benefit recognized for a position and the tax benefit claimed on a tax return is referred to as an unrecognized tax benefit. Potential interest and penalties associated with such uncertain tax positions are recorded as a component of income tax expense.

Net loss per common share

The Company computes basic net loss per common share by dividing net loss attributable to common stockholders by the weighted average number of common shares outstanding. During periods where the Company earns net income, the Company allocates participating securities a proportional share of net income determined by dividing total weighted average participating securities by the sum of the total weighted average common shares and participating securities (the “two-class method”). The Company’s preferred stock and vested restricted stock participate in any dividends declared by the Company and are therefore considered to be participating securities. Participating securities have the effect of diluting both basic and diluted earnings per share during periods of income. During periods where the Company incurred net loss, the Company allocates no loss to participating securities because they have no contractual obligation to share in the losses of the Company. The Company computes diluted net loss per common share after giving consideration to the dilutive effect of stock options, warrants and unvested restricted stock that are outstanding during the period, except where such non-participating securities would be anti-dilutive.

Comprehensive loss

The Company has no comprehensive loss items other than net loss.

Guarantees and indemnifications

The Company is not a guarantor under any agreements.

The Company leases office space under an operating lease. The Company has standard indemnification arrangements under these leases that require the Company to indemnify the landlord against losses, liabilities, and claims incurred in connection with the premises covered by the Company’s lease, the Company’s use of the premises, property damage or personal injury, and breach of the agreement.

Through December 31, 2014, the Company had not experienced any losses related to this indemnification obligation and no claims with respect thereto were outstanding. The Company does not expect material claims related to this indemnification obligation, and consequently, concluded that the fair value of this obligation is negligible and no related liabilities were established.

The Company has indemnified, under pre-determined conditions and limitations, a counterparty for infringement of third-party intellectual property rights by the Company. The Company does not believe, based on information available, that it is probable that any material amounts will be paid under these indemnification provisions.

As permitted under Delaware law, the Company indemnifies its officers, directors, and employees for certain events or occurrences while the officer or director is, or was, serving at the Company’s request in such capacity. The term of the indemnification is for the officer’s or director’s lifetime.

Recent Accounting Pronouncements

In June 2014, the FASB issued Accounting Standards Update No. 2014-12, Compensation—Stock Compensation (ASU 2014-12). The guidance provides clarification regarding the accounting for equity awards where a performance target that affects vesting and that could be achieved after the requisite service period should be accounted for as a performance condition. Compensation cost would be recognized over the required service period, if it is probable that the performance condition will be achieved. The Company does not expect the standard will have an impact on the Company’s consolidated financial statements and related disclosures since the Company currently accounting for these awards in a manner consistent with the new guidance.

Net_Loss_Per_Share_Attributabl

Net Loss Per Share Attributable to Common Stockholders	12 Months Ended
Net Loss Per Share Attributable to Common Stockholders	Dec. 31, 2014
Earnings Per Share [Abstract]
Net Loss Per Share Attributable to Common Stockholders	3. Net Loss Per Share Attributable to Common Stockholders

	The following table summarizes the computation of basic and diluted net loss per share attributable to common stockholders of the Company (in thousands, except share and per share data):



		YEARS ENDED DECEMBER 31,
		2014			2013			2012
	Net loss	$	(47,939	)	$	(18,518	)	$	(10,121	)
	Accretion of preferred stock issuance costs to redemption value		(204	)		(121	)		(61	)
	Accrued dividends on preferred stock		—			(2,267	)		(4,036	)

	Net loss attributable to common stockholders—basic and diluted	$	(48,143	)	$	(20,906	)	$	(14,218	)

	Weighted-average number of common shares—basic and diluted		16,070,054			29,463			27,554
	Net loss per share attributable to common stockholders—basic and diluted	$	(3.00	)	$	(709.57	)	$	(516.00	)

	The following potentially dilutive securities outstanding, prior to the application of the treasury stock method or if-converted method, have been excluded from the computation of diluted weighted-average common shares outstanding, because such securities had an anti-dilutive impact due to the losses reported:



		FOR THE YEARS ENDING DECEMBER 31,
		2014			2013			2012
	Options to purchase common stock		2,764,144			403,959			25,512
	Warrants to purchase common stock		80,722			2,198			2,198
	Warrants to purchase redeemable convertible preferred stock		12,763			91,543			47,400
	Redeemable convertible preferred stock		1,015,426			5,634,458			2,018,017
	Unvested restricted stock		92,932			7			152

Heldtomaturity_Investments

Held-to-maturity Investments	12 Months Ended
Held-to-maturity Investments	Dec. 31, 2014
Text Block [Abstract]
Held-to-maturity Investments	4. Held-to-maturity Investments

	The Company invests its excess cash balances in short-term and long-term fixed-income investments. The Company determines the appropriate classification of investments at the time of purchase and re-evaluates such designation as of each balance sheet date. Debt securities carried at amortized cost are classified as held-to-maturity when the Company has the positive intent and ability to hold the securities to maturity.

	The following tables provide information relating to held-to-maturity investments at December 31, 2014:



		Amortized		Gross		Gross			Fair
		Cost		Unrealized		Unrealized			Value
				Gains		Losses
	Held-to-maturity investments
	U.S. Government treasury and agency securities	$	72,556	$	2	$	(26	)	$	72,532

	Total held-to-maturity investments	$	72,556	$	2	$	(26	)	$	72,532



	The amortized cost and fair value of held-to-maturity investments by contractual maturities at December 31, 2014, are as follows:




		Held-to-Maturity
		Amortized		Fair
		Cost		Value
	Maturing in one year or less	$	70,055	$	70,030
	Maturing after one year through two years		2,501		2,502

	Total	$	72,556	$	72,532

Prepaid_Expenses_and_Other_Cur

Prepaid Expenses and Other Current Assets	12 Months Ended
Prepaid Expenses and Other Current Assets	Dec. 31, 2014
Deferred Costs, Capitalized, Prepaid, and Other Assets Disclosure [Abstract]
Prepaid Expenses and Other Current Assets	5. Prepaid Expenses and Other Current Assets

	Prepaid expenses and other current assets consists of the following:



		AS OF DECEMBER 31,
		2014		2013
	Prepaid expenses	$	1,115	$	261
	Interest receivable		55		—
	Debt issuance costs		—		12
	Deferred stock issuance costs		—		1,784
	Other current assets		24		1

	Prepaid expenses and other current assets	$	1,194	$	2,058

Property_and_Equipment_Net

Property and Equipment, Net	12 Months Ended
Property and Equipment, Net	Dec. 31, 2014
Property, Plant and Equipment [Abstract]
Property and Equipment, Net	6. Property and Equipment, Net

	Property and equipment, net consists of the following:



		AS OF DECEMBER 31,
		2014			2013
	Office and computer equipment	$	503		$	253
	Laboratory equipment		3,209			1,988
	Leasehold improvements		887			670
	Furniture and fixtures		624			299

	Property and equipment—at cost		5,223			3,210
	Less accumulated depreciation		(3,058	)		(2,333	)

	Property and equipment—net	$	2,165		$	877


	Depreciation expense for the years ended December 31, 2014, 2013 and 2012 was $725, $523 and, $551, respectively.

Longterm_Debt

Long-term Debt	12 Months Ended
	Dec. 31, 2014
Debt Disclosure [Abstract]
Long-term Debt	7. Long-term Debt

	On March 26, 2009, the Company entered into a loan and security agreement with an independent finance company, Hercules Technology II, LP (Hercules), for up to $7,000 (Hercules loan). The Hercules loan, which has since been repaid, was collateralized by a security interest in all tangible assets. On May 28, 2010, the Company and Hercules executed an amendment to the loan and security agreement to defer $653 in principal payments originally due in June 2010 through August 2010, and amortized such deferred principal amounts equally over the remaining payment term beginning in September 2010. On June 28, 2011, the Company and Hercules executed a second amendment to the loan and security agreement, which increased the maximum loan amount to $12,000. Upon execution of the second amendment, the Company drew a $7,000 advance, a portion of which the Company used to repay the outstanding balance of principal and interest under the original loan and security agreement. On December 15, 2011, the Company drew down the remaining $5,000. Interest was payable monthly and principal was to be repaid in equal monthly installments beginning April 1, 2012 through January 2, 2015. The applicable annual interest rate was 10.15% at December 31, 2013. On April 7, 2014, the Company repaid the remaining amount of the Hercules loan in full in a total amount of $3,590.

	The unamortized discount related to the Hercules loan of $143 was reflected as a loss on extinguishment of the Hercules loan in the accompanying statement of operations, when the loan was repaid.

	In connection with the Hercules loan, the Company issued warrants to Hercules for the purchase of an aggregate of 21,000 shares of the Series A preferred stock and 26,400 shares of the Series B preferred stock each at an exercise price of $25.00 per share. Immediately prior to the closing of the IPO on February 4, 2014, all of the outstanding shares of the Series A, Series B and Series C preferred stock were automatically converted into shares of common stock on an one-for-one basis. On February 11, 2014, Hercules net exercised the warrants in exchange for a total of 12,702 shares of the Company’s common stock. The fair value of the warrants was classified as a liability in the accompanying consolidated balance sheet as of December 31, 2013. After the conversion of Series A and Series B preferred stock, the fair value of the warrants was reclassified as a part of stockholders’ equity / (deficit). The remeasurement of the liability continued until the date of the closing of the IPO. The estimated fair value of the outstanding Hercules warrants as of the IPO closing date was $765 and $94 as of December 31, 2013. The estimated fair values were determined using the Black-Scholes option-pricing model with the following assumptions:



		FEBRUARY 4,			DECEMBER 31,
		2014			2013
	Stock price	$	32.66			$9.09
	Expected option term (in years)		3			3.6 – 4.1
	Expected volatility		62	%		59 – 64	%
	Risk-free interest rate		0.7	%		1.0 – 1.2	%
	Expected dividend yield		0	%		0	%

	There were no Series A and Series B warrants outstanding at December 31, 2014.

	The adjustment to this preferred stock warrant liability related to the Hercules warrants was recorded in other income (expense) and amounted to $(671) and $237 for the years ended December 31, 2014 and 2013, respectively.

Bridge_Loan_Financing

Bridge Loan Financing	12 Months Ended
	Dec. 31, 2014
Debt Disclosure [Abstract]
Bridge Loan Financing	8. Bridge Loan Financing

	On June 26, 2013, the Company issued convertible notes and warrants for the purchase of preferred stock in the next qualified financing to then existing preferred and common stockholders for approximately $3,000 (bridge loan financing). The notes were due to mature on June 26, 2018, unless previously converted or repaid. In the event the Company were to obtain financing through the issuance of equity securities, including an IPO, prior to the maturity date, the notes would automatically convert into shares of the equity issued in such a qualified financing and at the lowest price per share of the equity securities issued and sold in that financing. If a liquidation event were to occur prior to June 26, 2018, the notes could be converted at the option of the holder for an amount equal to two times the outstanding principal balance of the notes in cash or the issuance of Series B preferred stock equal to the outstanding principal balance of the notes divided by 0.9. The notes had an interest rate of 7% per year, payable at the earlier of the notes’ conversion or maturity.

	The purpose of this bridge loan financing was to provide operating cash to the Company until it completed the issuance of the Series C in July 2013, at which time the notes were converted into 428,526 shares of Series C preferred shares at $7.00 per share and the related accrued interest of $20 became payable.



	In connection with the issuance of the bridge loan financing, the Company issued warrants for the purchase of an aggregate of 85,703 shares of the Company’s preferred stock in the next qualified financing at an exercise price of $7.00 per share. The warrants are immediately exercisable and expire 5 years from the date of issuance. The Company estimated the fair value of the warrants at the date of issuance to be $324. The fair value of the warrants was recorded as a discount to the convertible notes upon issuance, and was initially classified as a liability in the accompanying consolidated balance sheets. The issuance date fair value was determined using the Black-Scholes option-pricing model with the following assumptions: risk-free interest rate of approximately 1.2%, expected life of 5 years, volatility of 64%, and no expected dividends. After the conversion of Series C preferred stock, the fair value of the warrants related to Series C preferred stock outstanding immediately prior to the closing of the IPO was reclassified as a part of stockholders’ equity (deficit). The remeasurement of the Series C preferred stock warrant liability continued until the closing date of the IPO.

	The estimated fair value of the outstanding Series C warrants at of the IPO closing date and December 31, 2013 was $2,323 and $435, respectively. The estimated fair values were determined using the Black-Scholes option-pricing model with the following assumptions:



		FEBRUARY 4,			DECEMBER 31,
		2014			2013
	Stock price	$	32.66		$	8.84
	Expected option term (in years)		4.4			4.5
	Expected volatility		65	%		64	%
	Risk-free interest rate		1.5	%		1.3	%
	Expected dividend yield		0	%		0	%

	There were no Series C warrants outstanding at December 31, 2014. As of the closing of the IPO on February 4, 2014, the conversion of Series C preferred stock into common stock triggered the conversion all Series C warrants into common share warrants. As of December 31, 2014 there were 85,703 common share warrants outstanding.

	The adjustment to this preferred stock warrant liability was recorded in other income (expense) and for the years ended December 31, 2014, and 2013 amounted to $(1,888) and $(111), respectively.

	The bridge loan discount was amortized to interest expense using the effective interest method over the loan repayment term, and the unamortized discount of $318 was reflected as a loss on extinguishment of the bridge loan in the accompanying statement of operations, when the notes were converted to Series C in July 2013.

Revenue

Revenue	12 Months Ended
Revenue	Dec. 31, 2014
Organization, Consolidation and Presentation of Financial Statements [Abstract]
Revenue	9. Revenue

	In December 2009, the Company entered into a research collaboration and license agreement with Kyowa Hakko Kirin Co., Ltd. (KHK) for the research, development and commercialization of drug delivery systems and DsiRNA pharmaceuticals for therapeutic targets primarily in oncology. The Company granted KHK an exclusive, worldwide, royalty-bearing and sub-licensable license to the DsiRNA and drug delivery technologies and intellectual property for certain selected DsiRNA-based compounds. Under the agreement, KHK is responsible for activities to develop, manufacture and commercialize the selected DsiRNA-based compounds and pharmaceutical products containing such compounds.

	Since the initiation of the research collaboration and license agreement two target programs, including the initial target KRAS, have been nominated by KHK for formal development studies. Both target programs utilize the Company’s specific RNAi-inducing double-stranded DsiRNA molecules and a lipid nanoparticle drug delivery system proprietary to KHK.

	The Company is entitled to receive up to $110,000 in regulatory, clinical and commercialization milestone payments, and royalties on net sales of each product candidate under the KHK agreement.



	The deliverables at the effective date of the agreement include delivery of intellectual property, conducting the KRAS research and development program, and providing KHK the exclusive option right to reserve additional targets. The Company concluded the performance of additional research for each additional target, if exercised by KHK, is not a deliverable of the agreement at inception because it is a substantive option and is not priced at a significant and incremental discount.

	The performance period is the expected period over which the services of the combined unit are performed which spans from the contract inception through the end of 2011. The Company received cash payments of $5,000 and $1,000 in 2013 and 2012, respectively, under the research collaboration and license agreement with KHK, which amounts included license fees, option exercise fees and research funding.

	The Company has no further obligations under the research collaboration and license agreement related to the KRAS target or the additional target.

	The Company recognized option and milestone payments of $0 and $7,015 in 2013 and 2012, respectively, related to the exercise of substantive options and milestones when the milestone and options were achieved and performance related to such milestones was completed.

	The Company has not recognized or received cash payments from partners during the year ended December 31, 2014.

Redeemable_Convertible_Preferr

Redeemable Convertible Preferred Stock

12 Months Ended

Dec. 31, 2014

Text Block [Abstract]

Redeemable Convertible Preferred Stock

10. Redeemable Convertible Preferred Stock

The consummation of the IPO on February 4, 2014 resulted in the conversion of all of the shares of the Company’s Series A, Series B and Series C preferred stock into shares of common stock. Each share of Series A, Series B and Series C preferred stock was automatically converted into common stock on a one-for-one basis.

Prior to conversion, the Company had classified the preferred stock outside of stockholders’ equity / (deficit) because the preferred stock contained redemption features that were not solely within the Company’s control.

There was no preferred stock issued or outstanding as of December 31, 2014.

Preferred stock consisted of the following as of December 31, 2013:

PREFERRED

ISSUANCE

PREFERRED

REDEMPTION

CARRYING

COMMON

DATES

VALUE/

VALUE

STOCK

AUTHORIZED

ISSUED AND

LIQUIDATION

ISSUABLE

OUTSTANDING

PREFERENCE

UPON

CONVERSION

Series A

880,000

Apr-07

9,999

Jul-07

19,999

Nov-07

489,999

Jul-08

335,999

855,996

21,400

855,996

Series B

1,190,000

Aug-10

992,021

September 2010

10,000

Oct-10

160,000

1,162,021

29,050

1,162,021

Series C

9,000,000

Jul-13

8,571,417

60,000

59,796

8,571,417

Total

11,070,000

10,589,434

110,450

110,246

10,589,434

The Series A, Series B and Series C preferred stock had the following rights and preferences:

Voting

The holders of Series A, Series B and Series C were entitled to one vote per common share equivalent on all matters voted on by holders of common stock.

Dividends

The holders of Series A and Series B were entitled to receive cumulative dividends at the rate of $2.00 per share per annum in preference to any dividends on common stock, payable only when and if declared by the Board of Directors. Dividends accrued daily in arrears and are not compounded, whether or not declared by the Board of Directors. As of December 31, 2013, no dividends had been declared.

Concurrent with the July 2013 Series C financing, the provisions relating to the Series A and Series B dividends were modified such that Series A and Series B dividends would accrue only if declared by the Board of Directors. On July 25, 2013, the carrying amount of the Series A and Series B preferred stock immediately preceding the modification included $16,170 of accrued dividends, which were cancelled and recorded as additional paid-in capital. If dividends were declared, the holders of Series C preferred stock were entitled to accrue dividends at the rate of $0.56 per share per annum in preference to any dividends on Series A, Series B or common stock. As of December 31, 2013, the Company had accrued no cumulative dividends and no dividends had been declared.

Liquidation rights

Prior to the IPO, and in the event of any liquidation, dissolution or winding-up of the Company, the Series C ranked senior to the Series A and Series B and the holders of Series C would have been entitled to receive their original purchase price together with all accumulated but unpaid dividends prior to any distributions being made to Series A and Series B. In the event of any liquidation, dissolution or winding-up of the Company, the Series A and Series B ranked senior to the Company’s common stock and the holders of Series A and Series B preferred stocks would have been entitled to receive their original purchase price together with all accumulated but unpaid dividends prior to any distributions being made to common stock. Upon completion of the payment of principal and declared but unpaid dividends to the holders of Series A and Series B preferred stocks, all of the remaining assets would have been distributed among the holders of Series A and Series B and common stock pro rata based on the number of shares of common stock held by each, assuming full conversion of all outstanding shares of Series A and Series B preferred stocks.

Conversion

Prior to the IPO, each Series A and Series B preferred stock was convertible at the option of the holder at any time after issuance into the number of fully paid and nonassessable shares of common stock as determined by dividing the original issuance price of $25.00 per share, plus any declared and unpaid dividends thereon by the conversion price in effect on the date the certificate is surrendered for conversion. The initial conversion price was $25.00 per share and was subject to adjustment for certain dilutive events, as defined. Each Series A and Series B preferred stock would be automatically be converted into one share of common stock in the event of an initial public offering that results in minimum net proceeds to the Company of $30,000 or at the election of the holders of at least 60% of the then outstanding Series A and Series B preferred stock. Each share of Series C was convertible at the option of the holder at any time after issuance into the number of fully paid and nonassessable shares of common stock as determined by dividing the original issuance price of $7.00 per share, plus any declared and unpaid dividends thereon by the conversion price in effect on the date the certificate is surrendered for conversion. The initial conversion price was $7.00 per share and was subject to adjustment for certain dilutive events, as defined. In July 2013, the terms of the Series A, Series B and Series C were amended such that each share of Series A, Series B and Series C would automatically be converted into one share of common stock in the event of an IPO that results in minimum net proceeds to the Company of $60,000 or at the election of the holders of at least a majority of the then outstanding shares of preferred stock, the majority of which includes at least 74% of the holders of the then outstanding shares of Series C. As of the closing of the IPO on February 4, 2014, the conversion of Series A, Series B and Series C preferred stock into common stock was triggered. All Series A, B and C preferred stock were converted into common stock.

Redemption

Prior to the IPO, and at any time on or after April 1, 2015, and at the request of the holders of not less than 60% of the then outstanding Series A and Series B preferred stock, the Company would have redeemed all of the then outstanding Series A and Series B preferred stock at an amount equal to the original issue price, plus all accumulated but unpaid dividends on such Series A and Series B preferred stock in three equal annual installments beginning on the redemption date.

Prior to the IPO, the Company was accreting the carrying value of Series A and Series B preferred stock to redemption value over the period from issuance to the earliest redemption date of April 1, 2015. The accretion amounts were recorded as an increase to the carrying value of the Series A and Series B preferred stock with a corresponding charge to additional paid-in capital, if any, and then to accumulated deficit.

Concurrent with the July 2013 issuance of Series C preferred stock, the Series A and Series B redemption date was extended to March 1, 2019. At any time on or after March 1, 2019 and at the request of the holders of at least a majority of the then outstanding shares of preferred stock, the majority of which includes at least 74% of the holders of the then outstanding shares of Series C, the Company would have redeemed all of the then outstanding shares of preferred stock at an amount equal to the original issue price, plus all dividends declared but unpaid on such shares of preferred stock in three equal annual installments beginning on the redemption date.

Modification of Series A and Series B rights and preferences

The aforementioned modification of the Series A and Series B rights and preferences was reflected as a deemed contribution by the Series A and Series B preferred stockholders of their forfeited rights to the accrued dividends on the modification date. On July 25, 2013, the carrying amount of the Series A and Series B preferred stock immediately preceding the modification included $16,170 of accrued dividends. The resulting deemed contribution was recorded as a $16,170 increase to additional paid-in capital.

Common_Stock_and_Stock_Option_

Common Stock and Stock Option Plan	12 Months Ended
Common Stock and Stock Option Plan	Dec. 31, 2014
Text Block [Abstract]
Common Stock and Stock Option Plan	11. Common Stock and Stock Option Plan

	Common stock

	Prior to the IPO, voting, dividend and liquidation rights of the holders of the common stock were subject to and qualified by the rights, powers and preferences of the holders of the preferred stock.

	Voting

	The holders of the common stock are entitled to one vote for each share of common stock held at all meetings of stockholders. The number of authorized shares of common stock may be increased or decreased (but not below the number of shares thereof) by the affirmative vote of the holders of shares of capital stock of the Company representing a majority of the votes represented by all outstanding shares of capital stock of the Company entitled to vote.

	Stock option plan

	On July 1, 2007, the Board of Directors approved the 2007 Employee, Director, and Consultant Stock Plan, which provides for the grant of qualified incentive stock options, nonqualified stock options, and restricted stock to employees, directors, and nonemployees. On May 5, 2010, the Board of Directors approved the Fourth Amended and Restated 2007 Employee, Director, and Consultant Stock Plan, which authorizes further issuances of up to 30,254 shares of the Company’s common stock. On October 14, 2010, the Board of Directors approved the retirement of the 2007 Employee, Director and Consultant Stock Plan and adopted the 2010 Employee Director and Consultant Equity Incentive Plan (the “2010 Plan”). The 2010 Plan, as adopted, authorizes further issuances of up to 45,214 shares of the Company’s common stock. On February 9, 2012, the Board of Directors approved an amendment to the 2010 Plan to increase the number of shares authorized for purchase by 4,800 shares, thereby providing for the purchase of up to 49,014 shares of the Company’s common stock. On July 30, 2013, the Board of Directors approved an amendment to the 2010 Plan to increase the number of shares authorized for purchase by 1,715,851 shares, thereby providing for the purchase of up to 1,764,865 shares of the Company’s common stock. The stock options generally vest 25% after 12 months, followed by ratable vesting over 36 months and expire 10 years from the grant date. On January 14, 2014, the Board of Directors approved the retirement of the 2010 Employee, Director and Consultant Stock Plan and adopted the 2014 Performance Incentive Plan (the “2014 Plan”). The 2014 Plan, as adopted, authorizes the issuances of up to 1,900,000 shares of the Company’s common stock, with an additional 4% of the total outstanding common shares becoming available at each year ending December 31. The stock options generally vest 25% after 12 months, followed by ratable vesting over 36 months and expire 10 years from the grant date.

	The fair value of each stock option award is estimated on the date of grant using the Black-Scholes option-pricing model that uses the assumptions noted in the table below. Expected volatility for the Company’s common stock was determined based on an average of the historical volatility of a peer group of similar companies. The Company has limited stock option exercise information, as such; the expected term of stock options granted was calculated using the simplified method, which represents the average of the contractual term of the stock option and the weighted-average vesting period of the stock option. The assumed dividend yield is based upon the Company’s expectation of not paying dividends in the foreseeable future. The risk-free rate for periods within the expected life of the stock option is based upon the U.S. Treasury yield curve in effect at the time of grant.

	The assumptions used in the Black-Scholes option-pricing model for stock options granted during the years ended December 31, 2014, 2013 and 2012 are as follows:



			DECEMBER 31,
			2014				2013				2012
	Expected option term (in years)			5.5 – 6.3				6				6
	Expected volatility			64% – 65	%			64% – 69	%			64	%
	Risk-free interest rate			1.7% – 2.0	%			1.2% – 1.9	%			0.7	%
	Expected dividend yield			0	%			0	%			0	%

	The weighted-average grant date fair value of stock options granted during the years ended December 31, 2014, 2013 and 2012 was $9.62, $2.52, and $30.00 per share, respectively. As of December 31, 2014, there was $18,284 of unrecognized compensation cost related to unvested employee stock options which are expected to be recognized over a weighted-average period of approximately 3 years. The intrinsic value of stock options exercised was $3,241, $0 and $7 for the years ended December 31, 2014, 2013 and 2012, respectively. Cash received from stock option exercises for the year ended December 31, 2014 was $824.

	On September 24, 2013, the Board of Directors approved the repricing of all of the then outstanding 24,811 stock options, with an original per-share weighted average exercise price of $45.16, to a new per-share exercise price of $3.42, which represented the current per-share fair market value. The repricing was treated by the Company as an exchange of the original awards for new awards. The incremental fair value of the modification, which was $30, was recognized in the statement of operations in 2013, representing the value of vested awards.



	A summary of stock option activity for employee and nonemployee awards under the Plan are presented below:



			NUMBER				WEIGHTED-				WEIGHTED-
			OF				AVERAGE				AVERAGE
			OPTIONS				PRICE				REMAINING
							PER SHARE				CONTRACTUAL
											TERM (YEARS)
	OUTSTANDING—January 1, 2014			1,610,139			$	3.42				9.7
	Granted			2,311,306				15.96
	Exercised			(239,853	)			3.44
	Forfeited			(76,879	)			11.72

	OUTSTANDING—December 31, 2014			3,604,713				11.28				9.1

	EXERCISABLE—December 31, 2014			787,661				7.52				8.8
	Vested and expected to vest as of December 31, 2014			3,603,566			$	11.28				9.1

	Under the Company’s 2014 stock option plans, the Company has reserved 61,694 shares of common stock for future issuance at December 31, 2014.

	Stock options granted to nonemployees

	Stock-based compensation expense related to stock options granted to nonemployees is recognized as the consulting services are rendered, generally on a straight-line basis. The Company believes that the fair value of the stock options is more reliably measurable than the fair value of the services received. Compensation expense is subject to remeasurement until the options vest.

	There were no such grants in 2014 and 2012. In 2013, the Company granted stock options to purchase 132,500 shares of common stock to nonemployees with an initial fair value of $337. The Company has recorded a stock-based compensation expense of $1,887, $23 and a stock-based compensation credit of $19 during the years ended December 31, 2014, 2013 and 2012, respectively. Compensation expense is subject to remeasurement until the stock options vest. The assumptions used to estimate fair value were as follows:



			DECEMBER 31,
			2014				2013				2012
Stock price		$	9.37 – 41.12			$	7.42			$	0.06
Expected option term (in years)			0.25 – 6.86				7				7
Expected volatility			56% – 68	%			66	%			66	%
Risk-free interest rate			0.1% – 2.3	%			2.8	%			1.7	%
Expected dividend yield			0	%			0	%			0	%

As of December 31, 2014, there were 26,250 unvested stock options. The remaining unrecognized compensation cost related to unvested nonemployee stock options is dependent on the valuation inputs used on each remeasurement date and will be recognized over a weighted-average period of thirty months.

Restricted common stock

During 2007, the Company issued a total of 320 shares of the Company’s common stock to a scientific advisor of the Company as consideration for services. The fair value of these shares was $8 at the grant date. Restricted common stock, prior to vesting, are subject to repurchase and transfer restrictions that lapsed in November 2011. As of December 31, 2014 and 2013, there was no remaining restricted common stock subject to repurchase. The Company has recorded stock-based compensation expense of $211, $0 and $3 in 2014, 2013 and 2012, respectively, associated with the vesting of restricted stock awards. In June 2011, the Company issued 200 shares of common stock of the Company’s common stock to a scientific advisor of the Company at a purchase price of $50.00 per share. The fair value of the common stock was $10 at the vesting date. The restricted common stock, prior to vesting, is subject to repurchase and transfer restrictions that lapse ratably over 48 months beginning in July 2011. As of December 31, 2012, there were 125 common stock subject to repurchase at $50.00 per share. In July 2013, the Company repurchased 100 unvested restricted common stock from the scientific advisor at a price of $50.00 per share. During 2014, the Company issued a total of 44,000 shares of the Company’s restricted common stock. The fair value of these shares were $709 at the grant date.

As of December 31, 2014 the unrecognized compensation cost related to restricted common stock was $498. As of December 31, 2013, there was no unrecognized compensation cost related to restricted common stock. The total fair value of restricted stock awards that vested during the years ended December 31, 2014, 2013 and 2012 (measured on the date of vesting) was $57, $1 and $3

A summary of the Company’s restricted common stock is presented below:



		SHARES				WEIGHTED-
						AVERAGE
						GRANT DATE
						FAIR VALUE
						PER SHARE
Nonvested—January 1, 2014			—			$	—
Issued			44,000				16.3
Vested			(4,000	)			14.34

Nonvested—December 31, 2014			40,000			$	16.3


Stock-based compensation expense is classified in the statements of operations as follows:



		YEAR ENDED DECEMBER 31,
		2014				2013				2012
Research and development		$	4,183			$	124			$	14
General and administrative			4,054				371				111

Total		$	8,237			$	495			$	125

Fair_Value_Measurements

Fair Value Measurements	12 Months Ended
Fair Value Measurements	Dec. 31, 2014
Fair Value Disclosures [Abstract]
Fair Value Measurements	12. Fair Value Measurements

	Fair value is an exit price, representing the amount that would be received from the sale of an asset or paid to transfer a liability in an orderly transaction between market participants. As such, fair value is a market-based measurement that should be determined based on assumptions that market participants would use in pricing an asset or liability. As a basis for considering such assumption the accounting literature establishes a three-tier value hierarchy which prioritizes the inputs used in measuring fair value as follows: (Level 1) observable inputs, such as unadjusted quoted prices in active markets; (Level 2) inputs other than the quoted prices in active markets that are observable either directly or indirectly; and (Level 3) unobservable inputs for which there is little or no market data, which requires the Company to develop its own assumptions. This hierarchy requires the Company to use observable market data, when available, and to minimize the use of unobservable inputs when determining fair value.



	A summary of the Company’s assets that are measured or disclosed at fair value on a recurring basis as at December 31, 2014 and 2013 are presented below:



	Description	At December 31,			(Level 1)		(Level 2)		(Level 3)
	Description	2014			(Level 1)		(Level 2)		(Level 3)
	Cash equivalents	$	20,425		$	20,425	$	—	$	—
	Held-to-maturity investments
	U.S. treasury securities and government agency bonds		72,532			—		72,532		—
	Assets held in restriction
	Certificates of deposit		1,380			—		1,380		—

	Total	$	94,361		$	20,425	$	73,936	$	—


	Description	At December 31,			(Level 1)		(Level 2)		(Level 3)
	Description	2013			(Level 1)		(Level 2)		(Level 3)
	Cash equivalents	$	46,041		$	46,041	$	—	$	—
	Assets held in restriction
	Certificates of deposit		264			—		264		—
	Long-term debt		4,847			—		4,847		—
	Preferred stock warrant liability		529			—		—		529

	Total	$	51,681		$	46,041	$	5,111	$	529


	The Company’s cash equivalents, primarily money market accounts are classified within Level 1 of the fair value hierarchy because they are valued using quoted prices as of December 31, 2014 and 2013, respectively.

	The Company’s assets held in restriction bore interest at the prevailing market rates for instruments with similar characteristics and, accordingly, the carrying value of these instruments also approximated their fair value and the financial instruments were classified within Level 2 of the fair value hierarchy because the inputs to the fair value measurement are valued using observable inputs as of December 31, 2014 and 2013, respectively.

	The Company’s held-to-maturity investments bore interest at the prevailing market rates for instruments with similar characteristics. The financial instruments were classified within Level 2 of the fair value hierarchy because the inputs to the fair value measurement are valued using observable inputs as of December 31, 2014.

	The Company’s long-term debt bore an interest at the prevailing market rates for instruments with similar characteristics and, accordingly, the carrying value for these instruments also approximated their fair value. There was no debt outstanding as of December 31, 2014.

	For the years ended December 31, 2014 and 2013, there were no transfers between Level 1 and Level 2.

	The fair value of the preferred stock warrant liability was determined using the Black-Scholes option-pricing model for the years ended December 31, 2013 and 2012 and until the IPO conversion date of February 4, 2014. The fair value of the preferred stock warrant was based significantly on the fair value of the preferred stock, which was developed using unobservable inputs, and was classified within Level 3.

	After the closing of the IPO, the remaining preferred stock warrant liability was no longer subject to re-measurement as the warrants to purchase the Company’s preferred stock became warrants to purchase shares of the Company’s common stock. As of the IPO closing date, the fair value of the preferred stock warrants was based significantly on the fair value of the Company’s publicly traded common stock and other observable inputs and was reclassified to Level 2. There were no preferred stock warrants outstanding as of December 31, 2014.



	The following table provides a roll-forward of the Company’s liabilities measured at fair value on a recurring basis using unobservable inputs (Level 3):



	BALANCE—January 1, 2012	$	800
	Change in fair value of warrant liability		(469	)

	BALANCE—December 31, 2012		331
	Issuance of preferred stock warrants		324
	Change in fair value of warrant liability		(126	)

	BALANCE—December 31, 2013		529
	Change in fair value of warrant liability		2,559
	Transfers to Level 2		(3,088	)

	BALANCE—December 31, 2014	$	—


The preferred stock warrant liability will increase or decrease each period based on the fluctuations of the fair value of the underlying preferred stock. A significant fluctuation in the preferred stock fair value would not result in a material increase or decrease in the fair value of the preferred stock warrant liability.

Income_Taxes

Income Taxes	12 Months Ended
Income Taxes	Dec. 31, 2014
Income Tax Disclosure [Abstract]
Income Taxes	13. Income Taxes
	The Company has a current alternative minimum tax of $125 and deferred income tax expense for the year ended December 31, 2014, and no current or deferred income tax expense for 2013. The Company did not recorded a federal income tax provision or benefit for the year ended December 31, 2014 and 2013.
	The reconciliation between income taxes computed at the federal statutory income tax rate and the provision for (benefit from) income taxes is as follows:


		YEARS ENDED DECEMBER 31,
		2014			2013			2012
	Federal statutory rate		34	%		34	%		34	%
	Effect of:
	Change in valuation allowance		(32.0	)		(35.4	)		(33.2	)
	Research and development tax credit		0.8			2.5			(1.9	)
	Stock-based compensation		(2.6	)		(1.3	)		(0.4	)
	Other		(0.2	)		0.2			1.5

	Total		0.0	%		0.0	%		0.0	%

	The components of the deferred tax assets are as follows:


		As of December 31,
		2014			2013
	Deferred tax assets:
	Net operating loss carryforwards	$	41,846		$	27,314
	Capitalized research and development costs		2,646			1,465
	Research and development credit carryforwards		2,360			1,806
	Stock Compensation		2,479			—
	Depreciation and other costs		372			386

	Net deferred tax assets		49,703			30,971
	Valuation allowance		(49,703	)		(30,971	)

	Net deferred tax assets	$	—		$	—


	Management of the Company has evaluated the positive and negative evidence bearing upon the realizability of its net deferred tax assets and determined that it is more likely than not that the Company will not recognize the benefits of the net deferred tax assets. As a result, the Company has a valuation allowance at December 31, 2014 and 2013. The valuation allowance increased in 2014 and 2013 by $18,732 and $8,116, primarily due to the increase in the Company’s net operating loss carryforwards, capitalized costs and stock compensation. As of December 31, 2014, the Company has approximately $57,800 of federal and $53,358 of state net operating loss carryforwards, and $1,457 of federal and $902 of Massachusetts research and development credits that expire starting in 2028.
	Realization of the future tax benefits is dependent on many factors, including the Company’s ability to generate taxable income within the net operating loss carryforward period. Under the provisions of the Internal Revenue Code, certain substantial changes in the Company’s ownership, including a sale of the Company or significant changes in ownership due to sales of equity, may have limited, or may limit in the future, the amount of net operating loss carryforwards, which could be used annually to offset future taxable income.
	As of December 31, 2014, the Company had $1,216 of unrecognized tax benefits, of which $1,216 would affect income tax expense if recognized, before consideration of the Company’s valuation allowance. The Company does not expect the unrecognized tax benefits to change significantly over the next 12 months. The Company recognizes both interest and penalties associated with uncertain tax positions as a component of income tax expense. As of December 31, 2014 and 2013, the Company has not accrued any penalties or made provisions for interest.
	A reconciliation of the gross unrecognized tax benefit is as follows (in thousands):


		Year Ended
		December 31,
		2014			2013
	Unrecognized tax benefits at the beginning of the period	$	865		$	563
	Additions for current tax positions		220			113
	Changes for previous tax positions		131			189

	Unrecognized tax benefits at the end of the period	$	1,216		$	865

	The Company files income tax returns in the United States and Commonwealth of Massachusetts. The tax years 2007 through 2014 remain open to examination by these jurisdictions, as carryforward attributes generated in past years may be adjusted in a future period. The Company is not currently under examination by the Internal Revenue Service or any other jurisdiction for these years. The Company has not recorded any interest or penalties for unrecognized tax benefits since its inception.
	In December 2014, we licensed all of our non-U.S. intellectual property rights to a non-U.S. wholly-owned subsidiary.

Commitments_and_Contingencies

Commitments and Contingencies	12 Months Ended
	Dec. 31, 2014
Commitments and Contingencies Disclosure [Abstract]
Commitments and Contingencies	14. Commitments and Contingencies

	Facility lease

	The Company began leasing office and lab space in 2008 under a noncancelable operating lease agreement that, as amended on July 3, 2013, expires on November 30, 2016. The lease agreement provides for an increasing monthly payment over the lease term. Rent expense is recorded on the straight-line basis and, therefore, the Company had a deferred rent obligation in the amount of $67 and $105 as of December 31, 2014 and 2013, respectively. The Company is also required to maintain a letter of credit of $264 related to the operating lease, which the letter of credit is secured by a certificate of deposit. The certificate of deposit is shown as assets held in restriction on the accompanying balance sheets.



	On July 11, 2014, the Company executed a non-cancelable operating lease for office and laboratory space in Cambridge, Massachusetts. The lease agreement obligates the Company to future minimum payments totaling $9,498 over a six-year lease term. Rent expense is recorded on the straight-line basis and, therefore, the Company had a deferred rent obligation in the amount of $10 as of December 31, 2014. The lease commenced on December 1, 2014. As part of the lease agreement, the Company established a $1,116 letter of credit, secured by a certificate of deposit which is included in assets held in restriction at December 31, 2014.

	Rent expense was $741, $875 and $899 and the years ended December 31, 2014, 2013 and 2012, respectively.

	Under the current lease agreements, future minimum payments payable are approximately as follows:



	PERIOD ENDING DECEMBER 30		OPERATING
			LEASES
	2015		$	2,101
	2016			2,120
	2017			1,582
	2018			1,629
	2019 and beyond			3,259

	Total (1)		$	10,691



	-1	Of the $10,691 future minimum payments payable noted above, $1,194 is associated with an offsetting sublease. The yearly total for 2015 and 2016 are as follows; $610, and $584 respectively.

	City of Hope license agreement

	In September 2007, the Company entered into a license agreement with City of Hope, an independent academic research and medical center (the “Medical Center”). In consideration for the right to develop, manufacture, and commercialize products based on certain of the Medical Center’s intellectual property, the Company paid a one-time, non-refundable license fee and issued shares of common stock as consideration for the license.

	The Company is required to pay an annual license maintenance fee, reimburse the Medical Center for patent costs incurred, and an amount within the range of $5,000 to $10,000 upon the achievement of certain milestones, and royalties on future sales, if any. There were no sublicense and other fees accrued as at December 31, 2014, and 2013. The license agreement will remain in effect until the expiration of the last patents or copyrights licensed under the agreement or until all obligations under the agreement with respect to payment of milestones have terminated or expired. The Company may terminate the license agreement at any time upon 90 days written notice to the Medical Center. The Company recorded research and development expense, related to the agreement with the Medical Center, of $50, $500 and $1,500 in 2014, 2013 and 2012, respectively.

	Plant Bioscience Limited license agreement

	In September 2013, the Company entered into a commercial license agreement with Plant Bioscience Limited (PBL), pursuant to which PBL has granted to the Company a license to certain of its U.S. patents and patent applications to research, discover, develop, manufacture, sell, import and export, products incorporating one or more short RNA molecules (SRMs).

	The Company has paid PBL a one-time, non-refundable signature fee and will pay PBL a nomination fee for any additional SRMs nominated by the Company under the agreement. The Company is further obligated to pay PBL milestone payments upon achievement of certain clinical and regulatory milestones. During 2014, the Company paid $100 to PBL based on meeting a clinical milestone. In addition, PBL is entitled to receive royalties of any net sale revenue of any licensed product candidates sold by the Company.



	Tekmira Pharmaceuticals Corporation license agreement

	In November 2014, the Company signed a licensing and collaboration agreement with Tekmira Pharmaceuticals Corporation (Tekmira) to license Tekmira’s LNP delivery technology for exclusive use in the Company’s primary hyperoxaluria type 1 (PH1) development program. The Company will use Tekmira’s LNP technology to deliver DCR-PH1, for the treatment of PH1. As of December 31, 2014, the Company paid $3,000 in license fees. Tekmira is entitled to receive additional payments of $22 million in aggregate development milestones, plus a mid-single-digit royalty on future PH1 sales. This new partnership also includes a supply agreement with Tekmira providing clinical drug supply and regulatory support.

Employee_Benefit_Plan

Employee Benefit Plan	12 Months Ended
Employee Benefit Plan	Dec. 31, 2014
Postemployment Benefits [Abstract]
Employee Benefit Plan	15. Employee Benefit Plan
Employee Benefit Plan	The Company has a 401(k) retirement plan in which substantially all employees are eligible to participate. Eligible employees may elect to contribute up to the maximum limits, as set by the Internal Revenue Service, of their eligible compensation. The Company made discretionary plan contributions of $175, $105 and $127 in 2014, 2013 and 2012, respectively.

Quarterly_Financial_Data_Unaud

Quarterly Financial Data (Unaudited)	12 Months Ended
Quarterly Financial Data (Unaudited)	Dec. 31, 2014
Quarterly Financial Information Disclosure [Abstract]
Quarterly Financial Data (Unaudited)	16. Quarterly Financial Data (Unaudited)



		FIRST			SECOND			THIRD			FOURTH			TOTAL
		QUARTER			QUARTER			QUARTER			QUARTER			YEAR
	2014
	Revenue	$	—		$	—		$	—		$	—		$	—
	Net loss		(10,804	)		(11,355	)		(11,193	)		(14,587	)		(47,939	)
	Net loss attributable to common stockholders		(11,008	)		(11,355	)		(11,193	)		(14,587	)		(48,143	)
	Net loss per share attributable to common stockholders—basic and diluted	$	(1.02	)	$	(0.64	)	$	(0.63	)	$	(0.82	)	$	(3.00	)



		FIRST			SECOND			THIRD			FOURTH			TOTAL
		QUARTER			QUARTER			QUARTER			QUARTER			YEAR
	2013
	Revenue	$	—		$	—		$	—		$	—		$	—
	Net loss		(3,819	)		(3,786	)		(4,194	)		(6,719	)		(18,518	)
	Net loss attributable to common stockholders		(4,830	)		(4,808	)		(4,540	)		(6,728	)		(20,906	)
	Net loss per share attributable to common stockholders—basic and diluted	$	(172.80	)	$	(171.13	)	$	(161.55	)	$	(199.91	)	$	(709.57	)

	Full year amounts may not sum due to rounding.

Summary_of_Significant_Account1

Summary of Significant Accounting Policies (Policies)	12 Months Ended
	Dec. 31, 2014
Accounting Policies [Abstract]
Basis of presentation and consolidation	Basis of presentation and consolidation
	The accompanying consolidated financial statements have been prepared under accounting principles generally accepted in the United States of America, or GAAP, and include the accounts of the Company and its wholly-owned subsidiary. All intercompany balances and transactions have been eliminated in consolidation.
Significant judgments and estimates	Significant judgments and estimates
	The preparation of these financial statements is in conformity with GAAP which requires management to make estimates and assumptions that affect the reported amounts of assets and liabilities, and the disclosure of contingent assets and liabilities at the date of the consolidated financial statements, as well as the revenues and expenses incurred during the reporting periods. On an ongoing basis, the Company evaluate judgments and estimates, including those related to accrued expenses, revenue recognition, deferred revenue and stock-based compensation. The Company base estimates on historical experience and on various other factors that the Company believes are reasonable under the circumstances, the results of which form the basis for making judgments about the carrying value of assets and liabilities that are not apparent from other sources. Changes in estimates are reflected in reported results for the period in which they become known. Actual results could differ materially from those estimates.
Cash equivalents	Cash equivalents
	Cash equivalents include all highly liquid investments maturing within 90 days from the date of purchase. Cash equivalents consist of money market funds as of December 31, 2014 and 2013 and are valued at cost, plus accrued interest, which approximates fair value.
Held-to-Maturity Investments	Held-to-Maturity Investments

	The Company accounts for its investments in marketable securities in accordance with FASB ASC 320, Investments—Debt and Equity Securities. The Company determines the appropriate classification of investments at the time of purchase and re-evaluates such designation as of each balance sheet date. Debt securities carried at amortized cost are classified as held-to-maturity when the Company has the positive intent and ability to hold the securities to maturity. At December 31, 2014, all of the Company’s investments are classified as held-to-maturity.
Assets held in restriction	Assets held in restriction

	As of December 31, 2014, assets held in restriction are comprised of two certificates of deposit that mature annually, and secure the Company’s outstanding letters of credit of $1,116 and $264 for the operating leases for office and laboratory space. The letters of credit are required to be maintained throughout the term of the Company’s leases which expire on December 1, 2020 and November 30, 2016, respectively. As of December 31, 2013, assets held in restriction is comprised of one certificate of deposit securing the Company’s outstanding letter of credit of $264, as noted above.
Concentrations of credit risk	Concentrations of credit risk

	Financial instruments that subject the Company to significant concentrations of credit risk consist of cash and cash equivalents, assets held in restriction and held-to-maturity investments. All of the Company’s cash and cash equivalents, assets held in restriction and held-to-maturity investments are invested in money market funds or U.S. Treasury or agency securities that management believes to be of high credit quality. During 2012, one counterparty accounted for all of the Company’s revenue.
Deferred stock issuance costs	Deferred stock issuance costs

	Deferred stock issuance costs, which consisted of direct incremental legal and professional accounting fees relating to the Company’s IPO, totaling $1,784, were initially capitalized in 2013 and subsequently offset against IPO proceeds in 2014, when the offering became effective. No amounts were deferred as of December 31, 2014.
Property and equipment	Property and equipment
	Property and equipment are stated at cost. Major betterments are capitalized whereas expenditures for maintenance and repairs which do not improve or extend the life of the respective assets are charged to operations as incurred. Depreciation is provided using the straight-line method over the estimated useful lives:


	ASSET CATEGORY	USEFUL LIVES
	Office and computer equipment		3-5 years
	Laboratory equipment		5 years
	Furniture and fixtures		5 years
	Leasehold improvements		5 years or the remaining
			term of lease, if shorter
Impairment of long-lived assets	Impairment of long-lived assets
	Long-lived assets are reviewed for impairment whenever events or changes in circumstances indicate that the carrying amount of the asset may not be recoverable. When such events occur, the Company compares the carrying amounts of the assets to their undiscounted expected future cash flows. If this comparison indicates that there is an impairment, the amount of the impairment is calculated as the difference between the carrying value and fair value of the related asset. For the years ended December 31, 2014, 2013 and 2012, no impairments have been recorded.
Fair value of financial instruments	Fair value of financial instruments
	The carrying amounts of the Company’s financial instruments include cash equivalents, accounts payable, and other accrued expenses, approximate their fair values due to their short duration. Management believes that the Company’s long-term debt bears interest at the prevailing market rate for instruments with similar characteristics and, accordingly, the carrying value of long-term debt also approximates their fair value.
Segment and geographic information	Segment and geographic information

	Operating segments are defined as components (business activity from which it earns revenue and incurs expenses) of an enterprise about which discrete financial information is available and regularly reviewed by the chief operating decision maker in deciding how to allocate resources and in assessing performance. The Company, through its Chief Executive Officer in his role as chief operating decision maker, views its operations and manages its business as one operating segment. All material long-lived assets of the Company are located in the United States.
Revenue recognition	Revenue recognition
	The Company generates revenue from research collaboration and license agreements with third parties which contain multiple deliverables. The deliverables in the agreements include (a) the use of the Company’s technology and (b) research and development of product candidates. Such agreements may provide for consideration to the Company in the form of up-front payments, research and development services, milestone payments and royalties. Revenue is recognized when the following criteria have been met: (1) pervasive evidence of an arrangement exists; (2) delivery has occurred or services have been rendered and risk of loss has passed; (3) the seller’s price to the buyer is fixed or determinable; and (4) collectability is reasonably assured. Multiple-deliverable arrangements are analyzed to determine whether the deliverables can be separated or whether they must be accounted for as a single unit of accounting. When deliverables are separable, consideration received is allocated to the separate units of accounting based on the relative selling price method and the appropriate revenue recognition principles are applied to each unit.
	At the inception of each arrangement that includes payments for optional research or milestones, the Company evaluates whether each option or milestone is substantive and at risk to both parties on the basis of the contingent nature of the option or milestone. This evaluation includes an assessment of whether (1) the consideration is commensurate with either the entity’s performance to achieve the milestone or the enhancement of the value of the delivered item(s); (2) as a result of a specific outcome resulting from the entity’s performance to achieve the milestone; (3) the consideration relates solely to past performance; and (4) the consideration is reasonable relative to all of the deliverables and payment terms within the arrangement. Substantive options and milestones are recognized as revenue upon the achievement of the milestone, assuming all other revenue recognition criteria are met.
	When the Company determines that an arrangement should be accounted for as a single unit of accounting, it must determine the period over which the performance obligations will be performed and revenue will be recognized. The Company’s only revenue to date results from a research collaboration and license agreement entered into in December 2009. Non-refundable up-front license fees under the agreement were initially recorded as deferred revenue upon receipt and are being recognized as revenue over the Company’s performance period as defined in the agreement. Research and development service revenue is recognized over the research term as the research and development services are provided. The cost of such services is reflected in research and development costs in the period in which it is incurred.
	Royalty payments are recognized as revenue based on contract terms and reported sales of licensed products, when reported sales are reliably measurable and collectability is reasonably assured.
Research and development costs	Research and development costs
	Research and development costs consist of expenses incurred in performing research and development activities, including compensation and benefits for full-time research and development employees, an allocation of facility expenses, overhead expenses and other outside expenses. Research and development costs are expensed as incurred. Research and development costs that are paid in advance of performance are deferred as a prepaid expense and amortized over the service period as the services are provided.
Preferred stock warrant liability	Preferred stock warrant liability
	Freestanding warrants related to shares that are redeemable, contingently redeemable, or for purchases of preferred stock that are not indexed to the Company’s own stock are classified as a liability on the Company’s balance sheet. The preferred stock warrants were recorded at fair value, estimated using the Black-Scholes option-pricing model for the year ended December 31, 2013 and until the conversion date of February 4, 2014, and marked to market at each balance sheet date with changes in the fair value of the liability recorded in the statements of operations. After the closing of the IPO, the preferred stock warrants were no longer classified as a liability subject to remeasurement as the preferred stock warrants became warrants to purchase shares of common stock. The Company classified the liability as noncurrent at December 31, 2013. There were no preferred stock warrants outstanding as of December 31, 2014.
Deferred Rent	Deferred Rent
	Deferred rent consists of rent escalation payment terms, tenant improvement allowances and other incentives received from landlords related to the Company’s operating leases. Rent escalation represents the difference between actual operating lease payments due and straight-line rent expense, which is recorded by the Company over the term of the lease. Tenant improvement allowances and other incentives are recorded as deferred rent and amortized as a reduction of periodic rent expense, over the term of the applicable lease.
Redeemable convertible preferred stock	Redeemable convertible preferred stock

	The Company initially records preferred stock that may be redeemed at the option of the holder or based on the occurrence of events not under the Company’s control outside of stockholders’ equity (deficit) at the value of the proceeds received or fair value, if lower, net of issuance costs. Subsequently, if it is probable that the preferred stock will become redeemable, the Company adjusts the carrying value to the redemption value over the period from the issuance date to the earliest possible redemption date using the effective interest method. If it is not probable that the preferred stock will become redeemable, the Company does not adjust the carrying value. In the absence of retained earnings these accretion charges are recorded against additional paid-in-capital, if any, and then to accumulated deficit.
Common stock valuation	Common stock valuation

	Due to the absence of an active market for the Company’s common stock prior to the IPO, the Company utilized methodologies in accordance with the framework of the American Institute of Certified Public Accountants Technical Practice Aid, Valuation of Privately-Held Company Equity Securities Issued as Compensation , to estimate the fair value of its common stock. In determining the exercise prices for options granted, the Company has considered the fair value of the common stock as of the measurement date. The fair value of the common stock has been determined at each award grant date based upon a variety of factors, including the illiquid nature of the common stock, arm’s-length sales of the Company’s capital stock (including redeemable convertible preferred stock), the effect of the rights and preferences of the preferred stockholders, and the prospects of a liquidity event. Among other factors are the Company’s financial position and historical financial performance, the status of technological developments within the Company’s research, the composition and ability of the current research and management team, an evaluation or benchmark of the Company’s competition, and the current business climate in the marketplace. Significant changes to the key assumptions underlying the factors used could result in different fair values of common stock at each valuation date.
Stock-based compensation	Stock-based compensation
	The Company accounts for stock options granted as share-based payments at fair value, which is measured using the Black-Scholes option pricing model. The fair value measurement date for employee awards is the date of grant. The fair value measurement date for nonemployee awards is generally the date the performance of services is completed. Share-based compensation costs are recognized as expense over the requisite service period, which is generally the vesting period, on a straight-line basis for all time-vested awards.
	For performance-based stock awards, compensation costs are first recorded when the Company determines that the achievement of such performance conditions is deemed probable. This determination requires significant judgment by management. At the probable date, the Company records a cumulative expense catch-up, with the remaining compensation cost being amortized over the remaining vesting period.
	The Company accounts for restricted stock awards granted to employees at fair value, which is measured based upon the quoted closing market price per share on the date of grant, adjusted for assumed forfeitures. The compensation costs are recognized over the vesting period, commencing when the Company determines that it is probable that the awards will vest.
	Share-based payments to nonemployees are remeasured at each reporting date and recognized as services are rendered, generally on a straight-line basis. The Company believes that the fair value of these awards is more reliably measurable than the fair value of the services rendered.
Income taxes	Income taxes
	The Company records deferred tax assets and liabilities for the expected future tax consequences of temporary differences between the Company’s financial statement carrying amounts and the tax basis of assets and liabilities using enacted tax rates expected to be in effect in the years in which the differences are expected to reverse. A valuation allowance is provided to reduce the net deferred tax assets to the amount that will more likely than not be realized.
	The Company also assesses the probability that the positions taken or expected to be taken in its income tax returns will be sustained by taxing authorities. A “more likely than not” (more than 50 percent) recognition threshold must be met before a tax benefit can be recognized. Tax positions that are more likely than not to be sustained are reflected in the Company’s financial statements. Tax positions are measured as the largest amount of tax benefit that is greater than 50 percent likely of being realized upon settlement with a taxing authority that has full knowledge of all relevant information. The difference between the benefit recognized for a position and the tax benefit claimed on a tax return is referred to as an unrecognized tax benefit. Potential interest and penalties associated with such uncertain tax positions are recorded as a component of income tax expense.
Net loss per common share	Net loss per common share
	The Company computes basic net loss per common share by dividing net loss attributable to common stockholders by the weighted average number of common shares outstanding. During periods where the Company earns net income, the Company allocates participating securities a proportional share of net income determined by dividing total weighted average participating securities by the sum of the total weighted average common shares and participating securities (the “two-class method”). The Company’s preferred stock and vested restricted stock participate in any dividends declared by the Company and are therefore considered to be participating securities. Participating securities have the effect of diluting both basic and diluted earnings per share during periods of income. During periods where the Company incurred net loss, the Company allocates no loss to participating securities because they have no contractual obligation to share in the losses of the Company. The Company computes diluted net loss per common share after giving consideration to the dilutive effect of stock options, warrants and unvested restricted stock that are outstanding during the period, except where such non-participating securities would be anti-dilutive.
Comprehensive loss	Comprehensive loss
	The Company has no comprehensive loss items other than net loss.
Guarantees and indemnifications	Guarantees and indemnifications

	The Company is not a guarantor under any agreements.

	The Company leases office space under an operating lease. The Company has standard indemnification arrangements under these leases that require the Company to indemnify the landlord against losses, liabilities, and claims incurred in connection with the premises covered by the Company’s lease, the Company’s use of the premises, property damage or personal injury, and breach of the agreement.

	Through December 31, 2014, the Company had not experienced any losses related to this indemnification obligation and no claims with respect thereto were outstanding. The Company does not expect material claims related to this indemnification obligation, and consequently, concluded that the fair value of this obligation is negligible and no related liabilities were established.

	The Company has indemnified, under pre-determined conditions and limitations, a counterparty for infringement of third-party intellectual property rights by the Company. The Company does not believe, based on information available, that it is probable that any material amounts will be paid under these indemnification provisions.

	As permitted under Delaware law, the Company indemnifies its officers, directors, and employees for certain events or occurrences while the officer or director is, or was, serving at the Company’s request in such capacity. The term of the indemnification is for the officer’s or director’s lifetime.
Recent Accounting Pronouncements	Recent Accounting Pronouncements

	In June 2014, the FASB issued Accounting Standards Update No. 2014-12, Compensation—Stock Compensation (ASU 2014-12). The guidance provides clarification regarding the accounting for equity awards where a performance target that affects vesting and that could be achieved after the requisite service period should be accounted for as a performance condition. Compensation cost would be recognized over the required service period, if it is probable that the performance condition will be achieved. The Company does not expect the standard will have an impact on the Company’s consolidated financial statements and related disclosures since the Company currently accounting for these awards in a manner consistent with the new guidance.

Summary_of_Significant_Account2

Summary of Significant Accounting Policies (Tables)	12 Months Ended
Summary of Significant Accounting Policies (Tables)	Dec. 31, 2014
Accounting Policies [Abstract]
Estimated Useful Lives	Depreciation is provided using the straight-line method over the estimated useful lives:


	ASSET CATEGORY	USEFUL LIVES
	Office and computer equipment		3-5 years
	Laboratory equipment		5 years
	Furniture and fixtures		5 years
	Leasehold improvements		5 years or the remaining
	Leasehold improvements		term of lease, if shorter

Net_Loss_Per_Share_Attributabl1

Net Loss Per Share Attributable to Common Stockholders (Tables)	12 Months Ended
	Dec. 31, 2014
Earnings Per Share [Abstract]
Summary of Computation of Basic and Diluted Net Loss Per Share Attributable to Common Stockholders	The following table summarizes the computation of basic and diluted net loss per share attributable to common stockholders of the Company (in thousands, except share and per share data):



		YEARS ENDED DECEMBER 31,
		2014			2013			2012
	Net loss	$	(47,939	)	$	(18,518	)	$	(10,121	)
	Accretion of preferred stock issuance costs to redemption value		(204	)		(121	)		(61	)
	Accrued dividends on preferred stock		—			(2,267	)		(4,036	)

	Net loss attributable to common stockholders—basic and diluted	$	(48,143	)	$	(20,906	)	$	(14,218	)

	Weighted-average number of common shares—basic and diluted		16,070,054			29,463			27,554
	Net loss per share attributable to common stockholders—basic and diluted	$	(3.00	)	$	(709.57	)	$	(516.00	)
Schedule of Dilutive Securities Outstanding	The following potentially dilutive securities outstanding, prior to the application of the treasury stock method or if-converted method, have been excluded from the computation of diluted weighted-average common shares outstanding, because such securities had an anti-dilutive impact due to the losses reported:



		FOR THE YEARS ENDING DECEMBER 31,
		2014			2013			2012
	Options to purchase common stock		2,764,144			403,959			25,512
	Warrants to purchase common stock		80,722			2,198			2,198
	Warrants to purchase redeemable convertible preferred stock		12,763			91,543			47,400
	Redeemable convertible preferred stock		1,015,426			5,634,458			2,018,017
	Unvested restricted stock		92,932			7			152

Heldtomaturity_Investments_Tab

Held-to-maturity Investments (Tables)	12 Months Ended
Held-to-maturity Investments (Tables)	Dec. 31, 2014
Text Block [Abstract]
Schedule of Held-To-Maturity Investments	The following tables provide information relating to held-to-maturity investments at December 31, 2014:


		Amortized		Gross		Gross			Fair
		Cost		Unrealized		Unrealized			Value
				Gains		Losses
	Held-to-maturity investments
	U.S. Government treasury and agency securities	$	72,556	$	2	$	(26	)	$	72,532

	Total held-to-maturity investments	$	72,556	$	2	$	(26	)	$	72,532

Amortized Cost and Fair Value of Held-To-Maturity Investments	The amortized cost and fair value of held-to-maturity investments by contractual maturities at December 31, 2014, are as follows:


		Held-to-Maturity
		Amortized		Fair
		Cost		Value
	Maturing in one year or less	$	70,055	$	70,030
	Maturing after one year through two years		2,501		2,502

	Total	$	72,556	$	72,532

Prepaid_Expenses_and_Other_Cur1

Prepaid Expenses and Other Current Assets (Tables)	12 Months Ended
Prepaid Expenses and Other Current Assets (Tables)	Dec. 31, 2014
Deferred Costs, Capitalized, Prepaid, and Other Assets Disclosure [Abstract]
Summary of Prepaid Expenses and Other Current Assets	Prepaid expenses and other current assets consists of the following:



		AS OF DECEMBER 31,
		2014		2013
	Prepaid expenses	$	1,115	$	261
	Interest receivable		55		—
	Debt issuance costs		—		12
	Deferred stock issuance costs		—		1,784
	Other current assets		24		1

	Prepaid expenses and other current assets	$	1,194	$	2,058

Property_and_Equipment_Net_Tab

Property and Equipment, Net (Tables)	12 Months Ended
Property and Equipment, Net (Tables)	Dec. 31, 2014
Property, Plant and Equipment [Abstract]
Summary of Property and Equipment, Net	Property and equipment, net consists of the following:



		AS OF DECEMBER 31,
		2014			2013
	Office and computer equipment	$	503		$	253
	Laboratory equipment		3,209			1,988
	Leasehold improvements		887			670
	Furniture and fixtures		624			299

	Property and equipment—at cost		5,223			3,210
	Less accumulated depreciation		(3,058	)		(2,333	)

	Property and equipment—net	$	2,165		$	877

Longterm_Debt_Tables

Long-term Debt (Tables)	12 Months Ended
Long-term Debt (Tables)	Dec. 31, 2014
Summary of Estimated Fair Values of Outstanding Warrants Using Black-Scholes Option-Pricing Model	The estimated fair values were determined using the Black-Scholes option-pricing model with the following assumptions:



		FEBRUARY 4,			DECEMBER 31,
		2014			2013
	Stock price	$	32.66			$9.09
	Expected option term (in years)		3			3.6 – 4.1
	Expected volatility		62	%		59 – 64	%
	Risk-free interest rate		0.7	%		1.0 – 1.2	%
	Expected dividend yield		0	%		0	%
Series C Warrants [Member]
Summary of Estimated Fair Values of Outstanding Warrants Using Black-Scholes Option-Pricing Model	The estimated fair values were determined using the Black-Scholes option-pricing model with the following assumptions:


		FEBRUARY 4,			DECEMBER 31,
		2014			2013
	Stock price	$	32.66		$	8.84
	Expected option term (in years)		4.4			4.5
	Expected volatility		65	%		64	%
	Risk-free interest rate		1.5	%		1.3	%
	Expected dividend yield		0	%		0	%

Redeemable_Convertible_Preferr1

Redeemable Convertible Preferred Stock (Tables)	12 Months Ended
Redeemable Convertible Preferred Stock (Tables)	Dec. 31, 2014
Text Block [Abstract]
Preferred Stock	Preferred stock consisted of the following as of December 31, 2013:



		PREFERRED		ISSUANCE	PREFERRED		REDEMPTION		CARRYING		COMMON
		SHARES		DATES	SHARES		VALUE/		VALUE		STOCK
		AUTHORIZED			ISSUED AND		LIQUIDATION				ISSUABLE
					OUTSTANDING		PREFERENCE				UPON
											CONVERSION
	Series A		880,000	Apr-07		9,999
				Jul-07		19,999
				Nov-07		489,999
				Jul-08		335,999

						855,996	$	21,400	$	21,400		855,996
	Series B		1,190,000	Aug-10		992,021
				September 2010		10,000
				Oct-10		160,000

						1,162,021		29,050		29,050		1,162,021

	Series C		9,000,000	Jul-13		8,571,417		60,000		59,796		8,571,417

	Total		11,070,000			10,589,434	$	110,450	$	110,246		10,589,434

Common_Stock_and_Stock_Option_1

Common Stock and Stock Option Plan (Tables)	12 Months Ended
Common Stock and Stock Option Plan (Tables)	Dec. 31, 2014
Assumptions Used in the Black-Scholes Option-Pricing Model for Stock Options Granted	The assumptions used in the Black-Scholes option-pricing model for stock options granted during the years ended December 31, 2014, 2013 and 2012 are as follows:



		DECEMBER 31,
		2014			2013			2012
	Expected option term (in years)		5.5 – 6.3			6			6
	Expected volatility		64% – 65	%		64% – 69	%		64	%
	Risk-free interest rate		1.7% – 2.0	%		1.2% – 1.9	%		0.7	%
	Expected dividend yield		0	%		0	%		0	%
Stock Option Activity for Employee and Nonemployee	A summary of stock option activity for employee and nonemployee awards under the Plan are presented below:



		NUMBER			WEIGHTED-			WEIGHTED-
		OF			AVERAGE			AVERAGE
		OPTIONS			PRICE			REMAINING
					PER SHARE			CONTRACTUAL
								TERM (YEARS)
	OUTSTANDING—January 1, 2014		1,610,139		$	3.42			9.7
	Granted		2,311,306			15.96
	Exercised		(239,853	)		3.44
	Forfeited		(76,879	)		11.72

	OUTSTANDING—December 31, 2014		3,604,713			11.28			9.1

	EXERCISABLE—December 31, 2014		787,661			7.52			8.8
	Vested and expected to vest as of December 31, 2014		3,603,566		$	11.28			9.1
Summary of Company's Restricted Common Stock	A summary of the Company’s restricted common stock is presented below:



		SHARES			WEIGHTED-
					AVERAGE
					GRANT DATE
					FAIR VALUE
					PER SHARE
	Nonvested—January 1, 2014		—		$	—
	Issued		44,000			16.3
	Vested		(4,000	)		14.34

	Nonvested—December 31, 2014		40,000		$	16.3

Stock-Based Compensation Expense	Stock-based compensation expense is classified in the statements of operations as follows:



		YEAR ENDED DECEMBER 31,
		2014			2013			2012
	Research and development	$	4,183		$	124		$	14
	General and administrative		4,054			371			111

	Total	$	8,237		$	495		$	125
Stock Options Granted to Nonemployees [Member]
Assumptions Used in the Black-Scholes Option-Pricing Model for Stock Options Granted	The assumptions used to estimate fair value were as follows:



		DECEMBER 31,
		2014			2013			2012
	Stock price	$	9.37 – 41.12		$	7.42		$	0.06
	Expected option term (in years)		0.25 – 6.86			7			7
	Expected volatility		56% – 68	%		66	%		66	%
	Risk-free interest rate		0.1% – 2.3	%		2.8	%		1.7	%
	Expected dividend yield		0	%		0	%		0	%

Fair_Value_Measurements_Tables

Fair Value Measurements (Tables)	12 Months Ended
Fair Value Measurements (Tables)	Dec. 31, 2014
Fair Value Disclosures [Abstract]
Schedule of Roll-Forward of Company's Liabilities Measured at Fair Value on Recurring Basis Using Unobservable Inputs (Level 3)	The following table provides a roll-forward of the Company’s liabilities measured at fair value on a recurring basis using unobservable inputs (Level 3):



	BALANCE—January 1, 2012	$	800
	Change in fair value of warrant liability		(469	)

	BALANCE—December 31, 2012		331
	Issuance of preferred stock warrants		324
	Change in fair value of warrant liability		(126	)

	BALANCE—December 31, 2013		529
	Change in fair value of warrant liability		2,559
	Transfers to Level 2		(3,088	)

	BALANCE—December 31, 2014	$	—

Schedule of Assets Measured or Disclosed at Fair Value on Recurring Basis	A summary of the Company’s assets that are measured or disclosed at fair value on a recurring basis as at December 31, 2014 and 2013 are presented below:



	Description	At December 31,			(Level 1)		(Level 2)		(Level 3)
	Description	2014			(Level 1)		(Level 2)		(Level 3)
	Cash equivalents	$	20,425		$	20,425	$	—	$	—
	Held-to-maturity investments
	U.S. treasury securities and government agency bonds		72,532			—		72,532		—
	Assets held in restriction
	Certificates of deposit		1,380			—		1,380		—

	Total	$	94,361		$	20,425	$	73,936	$	—


	Description	At December 31,			(Level 1)		(Level 2)		(Level 3)
	Description	2013			(Level 1)		(Level 2)		(Level 3)
	Cash equivalents	$	46,041		$	46,041	$	—	$	—
	Assets held in restriction
	Certificates of deposit		264			—		264		—
	Long-term debt		4,847			—		4,847		—
	Preferred stock warrant liability		529			—		—		529

	Total	$	51,681		$	46,041	$	5,111	$	529

Income_Taxes_Tables

Income Taxes (Tables)	12 Months Ended
Income Taxes (Tables)	Dec. 31, 2014
Income Tax Disclosure [Abstract]
Reconciliation Between Income Taxes Computed at the Federal Statutory Income Tax Rate and the Provision	The reconciliation between income taxes computed at the federal statutory income tax rate and the provision for (benefit from) income taxes is as follows:


		YEARS ENDED DECEMBER 31,
		2014			2013			2012
	Federal statutory rate		34	%		34	%		34	%
	Effect of:
	Change in valuation allowance		(32.0	)		(35.4	)		(33.2	)
	Research and development tax credit		0.8			2.5			(1.9	)
	Stock-based compensation		(2.6	)		(1.3	)		(0.4	)
	Other		(0.2	)		0.2			1.5

	Total		0.0	%		0.0	%		0.0
The components of the Deferred Tax Assets	The components of the deferred tax assets are as follows:


		As of December 31,
		2014			2013
	Deferred tax assets:
	Net operating loss carryforwards	$	41,846		$	27,314
	Capitalized research and development costs		2,646			1,465
	Research and development credit carryforwards		2,360			1,806
	Stock Compensation		2,479			—
	Depreciation and other costs		372			386

	Net deferred tax assets		49,703			30,971
	Valuation allowance		(49,703	)		(30,971	)

	Net deferred tax assets	$	—		$	—

Unrecognized Tax Benefit	A reconciliation of the gross unrecognized tax benefit is as follows (in thousands):



		Year Ended
		December 31,
		2014			2013
	Unrecognized tax benefits at the beginning of the period	$	865		$	563
	Additions for current tax positions		220			113
	Changes for previous tax positions		131			189

	Unrecognized tax benefits at the end of the period	$	1,216		$	865

Commitments_and_Contingencies_

Commitments and Contingencies (Tables)	12 Months Ended
Commitments and Contingencies (Tables)	Dec. 31, 2014
Commitments and Contingencies Disclosure [Abstract]
Summary of Future Minimum Payments on Lease Agreements	Under the current lease agreements, future minimum payments payable are approximately as follows:



	PERIOD ENDING DECEMBER 30		OPERATING
	PERIOD ENDING DECEMBER 30		LEASES
	2015		$	2,101
	2016			2,120
	2017			1,582
	2018			1,629
	2019 and beyond			3,259

	Total (1)		$	10,691



	-1	Of the $10,691 future minimum payments payable noted above, $1,194 is associated with an offsetting sublease. The yearly total for 2015 and 2016 are as follows; $610, and $584 respectively.

Quarterly_Financial_Data_Unaud1

Quarterly Financial Data (Unaudited) (Tables)	12 Months Ended
Quarterly Financial Data (Unaudited) (Tables)	Dec. 31, 2014
Quarterly Financial Information Disclosure [Abstract]
Schedule of Quarterly Financial Data		FIRST				SECOND				THIRD				FOURTH				TOTAL
		QUARTER				QUARTER				QUARTER				QUARTER				YEAR
	2014
	Revenue		$	—			$	—			$	—			$	—			$	—
	Net loss			(10,804	)			(11,355	)			(11,193	)			(14,587	)			(47,939	)
	Net loss attributable to common stockholders			(11,008	)			(11,355	)			(11,193	)			(14,587	)			(48,143	)
	Net loss per share attributable to common stockholders—basic and diluted		$	(1.02	)		$	(0.64	)		$	(0.63	)		$	(0.82	)		$	(3.00	)



			FIRST				SECOND				THIRD				FOURTH				TOTAL
			QUARTER				QUARTER				QUARTER				QUARTER				YEAR
	2013
	Revenue		$	—			$	—			$	—			$	—			$	—
	Net loss			(3,819	)			(3,786	)			(4,194	)			(6,719	)			(18,518	)
	Net loss attributable to common stockholders			(4,830	)			(4,808	)			(4,540	)			(6,728	)			(20,906	)
	Net loss per share attributable to common stockholders—basic and diluted		$	(172.80	)		$	(171.13	)		$	(161.55	)		$	(199.91	)		$	(709.57	)

Nature_of_Business_and_Basis_o1

Nature of Business and Basis of Presentation - Additional Information (Detail) (USD $)	12 Months Ended		1 Months Ended
In Thousands, except Share data, unless otherwise specified	Dec. 31, 2014	Dec. 31, 2013	Feb. 28, 2014
Organization Consolidation And Presentation Of Financial Statements [Line Items]
Proceeds from issuance of common stock	$94,148
Underwriting discounts and commissions		1,399
Preferred stock conversion basis	In connection with the close of the IPO, all of the outstanding shares of Series A mandatorily redeemable, convertible preferred stock (Series A preferred stock), Series B mandatorily redeemable, convertible preferred stock (Series B preferred stock) and Series C mandatorily redeemable, convertible preferred stock (Series C preferred stock) were converted into shares of common stock on a one-for-one basis immediately prior to the closing of the IPO.
Common stock, shares issued	17,786,867	38,226
Common stock, shares outstanding	17,786,867	38,226
IPO [Member]
Organization Consolidation And Presentation Of Financial Statements [Line Items]
Common stock shares issued			6,900,000
Sale of stock price per share			$15
Proceeds from issuance of common stock			92,750
Underwriting discounts and commissions			7,245
Offering expenses			$3,506

Summary_of_Significant_Account3

Summary of Significant Accounting Policies - Additional Information (Detail) (USD $)	12 Months Ended
	Dec. 31, 2014	Dec. 31, 2013	Dec. 31, 2012
	Segment	Dec. 31, 2013	Dec. 31, 2012
Schedule Of Significant Accounting Policies [Line Items]
Assets held in restriction	$1,380,000	$264,000
Counterparty accounted for all of the Company's revenue and research and license agreement receivable	During 2012, one counterparty accounted for all of the Company's revenue.
Deferred IPO costs	0	1,784,000
Long-lived assets, impairment charge	0	0	0
Number of operating segment	1
Letter of Credit, 2014 Operating lease Agreement [Member]
Schedule Of Significant Accounting Policies [Line Items]
Assets held in restriction	1,116,000
Lease Expiration Date	1-Dec-20
Letters Of Credit,2008 Operating Lease Agreement [Member]
Schedule Of Significant Accounting Policies [Line Items]
Assets held in restriction	$264,000	$264,000
Lease Expiration Date	30-Nov-16
Preferred Stock [Member]
Schedule Of Significant Accounting Policies [Line Items]
Preferred stock warrants outstanding	0

Summary_of_Significant_Account4

Summary of Significant Accounting Policies - Estimated Useful Lives (Detail)	12 Months Ended
	Dec. 31, 2014
Office and Computer Equipment [Member] \| Minimum [Member]
Property, Plant and Equipment [Line Items]
Useful Lives	3 years
Office and Computer Equipment [Member] \| Maximum [Member]
Property, Plant and Equipment [Line Items]
Useful Lives	5 years
Laboratory Equipment [Member]
Property, Plant and Equipment [Line Items]
Useful Lives	5 years
Furniture and Fixtures [Member]
Property, Plant and Equipment [Line Items]
Useful Lives	5 years
Leasehold Improvements [Member]
Property, Plant and Equipment [Line Items]
Useful Lives	5 years or the remaining term of lease, if shorter

Net_Loss_Per_Share_Attributabl2

Net Loss Per Share Attributable to Common Stockholders - Summary of Computation of Basic and Diluted Net Loss Per Share Attributable to Common Stockholders (Detail) (USD $)	3 Months Ended								12 Months Ended
In Thousands, except Share data, unless otherwise specified	Dec. 31, 2014	Sep. 30, 2014	Jun. 30, 2014	Mar. 31, 2014	Dec. 31, 2013	Sep. 30, 2013	Jun. 30, 2013	Mar. 31, 2013	Dec. 31, 2014	Dec. 31, 2013	Dec. 31, 2012
Earnings Per Share [Abstract]
Net loss	($14,587)	($11,193)	($11,355)	($10,804)	($6,719)	($4,194)	($3,786)	($3,819)	($47,939)	($18,518)	($10,121)
Accretion of preferred stock issuance costs to redemption value									-204	-121	-61
Accrued dividends on preferred stock										-2,267	-4,036
Net loss attributable to common stockholders-basic and diluted									($48,143)	($20,906)	($14,218)
Weighted-average number of common shares-basic and diluted									16,070,054	29,463	27,554
Net loss per share attributable to common stockholders-basic and diluted	($0.82)	($0.63)	($0.64)	($1.02)	($199.91)	($161.55)	($171.13)	($172.80)	($3)	($709.57)	($516)

Net_Loss_Per_Share_Attributabl3

Net Loss Per Share Attributable to Common Stockholders - Schedule of Dilutive Securities Outstanding (Detail)	12 Months Ended
	Dec. 31, 2014	Dec. 31, 2013	Dec. 31, 2012
Options to Purchase Common Stock [Member]
Antidilutive Securities Excluded from Computation of Earnings Per Share [Line Items]
Anti-dilutive securities	2,764,144	403,959	25,512
Hercules Warrants [Member]
Antidilutive Securities Excluded from Computation of Earnings Per Share [Line Items]
Anti-dilutive securities	80,722	2,198	2,198
Warrants to Purchase Redeemable Convertible Preferred Stock [Member]
Antidilutive Securities Excluded from Computation of Earnings Per Share [Line Items]
Anti-dilutive securities	12,763	91,543	47,400
Redeemable Convertible Preferred Stock [Member]
Antidilutive Securities Excluded from Computation of Earnings Per Share [Line Items]
Anti-dilutive securities	1,015,426	5,634,458	2,018,017
Unvested Restricted Common Stock [Member]
Antidilutive Securities Excluded from Computation of Earnings Per Share [Line Items]
Anti-dilutive securities	92,932	7	152

HeldtoMaturity_Investments_Sch

Held-to-Maturity Investments - Schedule of Held-to-Maturity Investments (Detail) (USD $)	Dec. 31, 2014
In Thousands, unless otherwise specified	Dec. 31, 2014
Schedule of Held-to-maturity Securities [Line Items]
Held to Maturity, Amortized Cost, Total	$72,556
Held-to-maturity investments, Gross Unrealized Gains	2
Held-to-maturity investments, Gross Unrealized Losses	-26
Held-to-maturity investments, Fair Value, Total	72,532
U.S. Government Treasury and Agency Securities [Member]
Schedule of Held-to-maturity Securities [Line Items]
Held to Maturity, Amortized Cost, Total	72,556
Held-to-maturity investments, Gross Unrealized Gains	2
Held-to-maturity investments, Gross Unrealized Losses	-26
Held-to-maturity investments, Fair Value, Total	$72,532

HeldtoMaturityInvestments_Amor

Held-to-Maturity-Investments - Amortized Cost and Fair Value of Held-to-Maturity Investments (Detail) (USD $)	Dec. 31, 2014
In Thousands, unless otherwise specified	Dec. 31, 2014
Investments, Debt and Equity Securities [Abstract]
Held-to-Maturity securities, Maturing in one year or less, Amortized Cost	$70,055
Held-to-Maturity securities, Maturing after one year through two years, Amortized Cost	2,501
Held to Maturity, Amortized Cost, Total	72,556
Held-to-Maturity securities, Maturing in one year or less, Fair Value	70,030
Held-to-Maturity securities, Maturing after one year through two years, Fair Value	2,502
Held-to-Maturity, Fair Value, Total	$72,532

Prepaid_Expenses_and_Other_Cur2

Prepaid Expenses and Other Current Assets - Summary of Prepaid Expenses and Other Current Assets (Detail) (USD $)	12 Months Ended
In Thousands, unless otherwise specified	Dec. 31, 2013	Dec. 31, 2014
Deferred Costs, Capitalized, Prepaid, and Other Assets Disclosure [Abstract]
Prepaid expenses	$261	$1,115
Interest receivable		55
Debt issuance costs	12
Deferred stock issuance costs	1,784	0
Other current assets	1	24
Prepaid expenses and other current assets	$2,058	$1,194

Property_and_Equipment_Net_Sum

Property and Equipment, Net - Summary of Property and Equipment, Net (Detail) (USD $)	Dec. 31, 2014	Dec. 31, 2013
In Thousands, unless otherwise specified	Dec. 31, 2014	Dec. 31, 2013
Property, Plant and Equipment [Line Items]
Property and equipment-at cost	$5,223	$3,210
Less accumulated depreciation	-3,058	-2,333
Property and equipment-net	2,165	877
Office and Computer Equipment [Member]
Property, Plant and Equipment [Line Items]
Property and equipment-at cost	503	253
Laboratory Equipment [Member]
Property, Plant and Equipment [Line Items]
Property and equipment-at cost	3,209	1,988
Leasehold Improvements [Member]
Property, Plant and Equipment [Line Items]
Property and equipment-at cost	887	670
Furniture and Fixtures [Member]
Property, Plant and Equipment [Line Items]
Property and equipment-at cost	$624	$299

Property_and_Equipment_Net_Add

Property and Equipment, Net - Additional Information (Detail) (USD $)	12 Months Ended
In Thousands, unless otherwise specified	Dec. 31, 2014	Dec. 31, 2013	Dec. 31, 2012
Property, Plant and Equipment [Abstract]
Depreciation expense	$725	$523	$551

Longterm_Debt_Additional_Infor

Long-term Debt - Additional Information (Detail) (USD $)	0 Months Ended	12 Months Ended			0 Months Ended
Long-term Debt - Additional Information (Detail) (USD $)	28-May-10	Dec. 31, 2014	Dec. 31, 2013	Dec. 31, 2012	Dec. 15, 2011	Jun. 28, 2011	Apr. 07, 2014	Feb. 04, 2014	Mar. 26, 2009	Feb. 11, 2014
Debt Instrument [Line Items]
Loan and security agreement									$7,000,000
Loan and security agreement, principal payments	653,000
Principal payments due		June 2010 through August 2010
Loss on extinguishment of loan		-143,000	-318,000
Common stock issued, shares		17,786,867	38,226
Estimated fair value of outstanding warrants			94,000					765,000
Adjustment to preferred stock warrant liability recorded in other income (expense)		-2,559,000	126,000	469,000
Series A Warrants [Member]
Debt Instrument [Line Items]
Estimated fair value of outstanding warrants		0
Series B Warrants [Member]
Debt Instrument [Line Items]
Estimated fair value of outstanding warrants		0
Hercules Warrants [Member]
Debt Instrument [Line Items]
Adjustment to preferred stock warrant liability recorded in other income (expense)		-671,000	237,000
Second Amendment [Member]
Debt Instrument [Line Items]
Loan and security agreement						12,000,000
Loan drew in advance					5,000,000	7,000,000
Annual interest rate			10.15%
Principal monthly installments period		April 1, 2012 through January 2, 2015
Hercules Loan [Member]
Debt Instrument [Line Items]
Repayment of debt							$3,590,000
IPO closing date		4-Feb-14
Common stock issued, shares										12,702
Hercules Loan [Member] \| Series A Warrants [Member]
Debt Instrument [Line Items]
Warrants exercised, number of shares		21,000
Warrants exercised, exercise price		$25
Hercules Loan [Member] \| Series B Warrants [Member]
Debt Instrument [Line Items]
Warrants exercised, number of shares		26,400
Warrants exercised, exercise price		$25

Longterm_Debt_Summary_of_Estim

Long-term Debt - Summary of Estimated Fair Values of Outstanding Warrants Using Black-Scholes Option-Pricing Model (Detail) (Hercules Warrants [Member], USD $)	0 Months Ended	12 Months Ended
	Feb. 04, 2014	Dec. 31, 2013	Feb. 04, 2014
Fair Value Measurements, Recurring and Nonrecurring, Valuation Techniques [Line Items]
Stock price	$32.66	$9.09	$32.66
Expected option term (in years)	3 years
Expected volatility	62.00%
Risk-free interest rate	0.70%
Expected dividend yield	0.00%	0.00%
Minimum [Member]
Fair Value Measurements, Recurring and Nonrecurring, Valuation Techniques [Line Items]
Expected option term (in years)		3 years 7 months 6 days
Expected volatility		59.00%
Risk-free interest rate		1.00%
Maximum [Member]
Fair Value Measurements, Recurring and Nonrecurring, Valuation Techniques [Line Items]
Expected option term (in years)		4 years 1 month 6 days
Expected volatility		64.00%
Risk-free interest rate		1.20%

Bridge_Loan_Financing_Addition

Bridge Loan Financing - Additional Information (Detail) (USD $)	12 Months Ended			0 Months Ended		1 Months Ended
	Dec. 31, 2014	Dec. 31, 2013	Dec. 31, 2012	Feb. 04, 2014	Jun. 26, 2013	Jun. 26, 2013
Short-term Debt [Line Items]
Convertible notes issues		$3,000,000
Shares issued		10,589,434
Estimated fair value of warrants outstanding		94,000		765,000
Adjustment to preferred stock warrant liability recorded in other income (expense)	-2,559,000	126,000	469,000
Loss on extinguishment of debt	-143,000	-318,000
Common Stock [Member]
Short-term Debt [Line Items]
Shares issued	10,589,434
Series C Redeemable Convertible Preferred Stock [Member]
Short-term Debt [Line Items]
Shares issued	8,571,417
Series C Warrants [Member]
Short-term Debt [Line Items]
Risk free interest rate		1.30%		1.50%
Expected life		4 years 6 months		4 years 4 months 24 days
Volatility rate		64.00%		65.00%
Bridge Loan [Member]
Short-term Debt [Line Items]
Convertible notes issues					3,000,000
Maturity date of debt	26-Jun-18
Conversion basis	If a liquidation event were to occur prior to June 26, 2018, the notes could be converted at the option of the holder for an amount equal to two times the outstanding principal balance of the notes in cash or the issuance of Series B preferred stock equal to the outstanding principal balance of the notes divided by 0.9.
Interest rate					7.00%
Warrants to purchase stock, shares issued					85,703	85,703
Warrants to purchase stock, exercise price					$7	$7
Fair value of warrants granted					324,000	324,000
Risk free interest rate						1.20%
Expected life						5 years
Volatility rate						64.00%
Expected dividend						$0
Adjustment to preferred stock warrant liability recorded in other income (expense)	-1,888,000	-111,000
Loss on extinguishment of debt	-318,000
Bridge Loan [Member] \| Common Stock [Member]
Short-term Debt [Line Items]
Share warrants outstanding	85,703
Bridge Loan [Member] \| Series C Redeemable Convertible Preferred Stock [Member]
Short-term Debt [Line Items]
Shares issued					428,526	428,526
Conversion price					$7	$7
Accrued interest payable					20,000	20,000
Bridge Loan [Member] \| Series C Warrants [Member]
Short-term Debt [Line Items]
Estimated fair value of warrants outstanding		$435,000		2,323,000
Share warrants outstanding	0

Bridge_Loan_Financing_Summary_

Bridge Loan Financing - Summary of Estimated Fair Values of Outstanding Warrants Using Black-Scholes Option-Pricing Model (Detail) (Series C Warrants [Member], USD $)	0 Months Ended	12 Months Ended
	Feb. 04, 2014	Dec. 31, 2013	Feb. 04, 2014
Series C Warrants [Member]
Fair Value Measurements, Recurring and Nonrecurring, Valuation Techniques [Line Items]
Stock price	$32.66	$8.84	$32.66
Expected option term (in years)	4 years 4 months 24 days	4 years 6 months
Expected volatility	65.00%	64.00%
Risk-free interest rate	1.50%	1.30%
Expected dividend yield	0.00%	0.00%

Revenue_Additional_Information

Revenue - Additional Information (Detail) (KHK Agreement [Member], USD $)	12 Months Ended
	Dec. 31, 2013	Dec. 31, 2012	Dec. 31, 2014
Revenue Recognition, Multiple-deliverable Arrangements [Line Items]
Cash payment received	$5,000,000	$1,000,000
Option and milestone payments recognized	0	7,015,000
Deferred revenue			0
Maximum [Member]
Revenue Recognition, Multiple-deliverable Arrangements [Line Items]
Regulatory, clinical and commercialization milestone payments, and royalties on net sales entitled to receive			$110,000,000

Redeemable_Convertible_Preferr2

Redeemable Convertible Preferred Stock - Additional Information (Detail) (USD $)	0 Months Ended		12 Months Ended		1 Months Ended
	Dec. 31, 2013	Jul. 25, 2013	Dec. 31, 2013	Dec. 31, 2014	Jul. 31, 2013	Jun. 30, 2013	Jul. 24, 2013
Conversion of Stock [Line Items]
Preferred Stock issued	0		0	0
Preferred Stock outstanding	0		0	0
Dividends declared	$0
Accrued dividends, carrying amount							16,170,000
Increase in additional paid-in capital		16,170,000	16,170,000
Series A Redeemable Convertible Preferred Stock [Member]
Conversion of Stock [Line Items]
Cumulative dividends per share				$2
Series B Redeemable Convertible Preferred Stock [Member]
Conversion of Stock [Line Items]
Cumulative dividends per share				$2
Series C Redeemable Convertible Preferred Stock [Member]
Conversion of Stock [Line Items]
Cumulative dividends per share			$0.56
Share issuance price						7
Series C Redeemable Convertible Preferred Stock [Member] \| Conversion criteria, one \| Minimum [Member]
Conversion of Stock [Line Items]
Aggregate gross proceeds					60,000,000
Series C Redeemable Convertible Preferred Stock [Member] \| Conversion criteria, two
Conversion of Stock [Line Items]
Conversion basis, percentage of outstanding stock					74.00%
Series C Redeemable Convertible Preferred Stock [Member] \| Conversion criteria, two \| Minimum [Member]
Conversion of Stock [Line Items]
Conversion basis, percentage of outstanding stock						60.00%
Series A And Series B Preferred Stock
Conversion of Stock [Line Items]
Share issuance price						25
Preferred stock, redemption date				1-Apr-15
Series A And Series B Preferred Stock \| Conversion criteria, one \| Minimum [Member]
Conversion of Stock [Line Items]
Aggregate gross proceeds						30,000,000

Redeemable_Convertible_Preferr3

Redeemable Convertible Preferred Stock - Preferred Stock (Detail) (USD $)	Dec. 31, 2014	Dec. 31, 2013	Jul. 31, 2008	Nov. 30, 2007	Jul. 31, 2007	Apr. 30, 2007	Oct. 31, 2010	Sep. 30, 2010	Aug. 31, 2010	Jul. 31, 2013
In Thousands, except Share data, unless otherwise specified	Dec. 31, 2014	Dec. 31, 2013	Jul. 31, 2008	Nov. 30, 2007	Jul. 31, 2007	Apr. 30, 2007	Oct. 31, 2010	Sep. 30, 2010	Aug. 31, 2010	Jul. 31, 2013
Temporary Equity [Line Items]
PREFERRED SHARES AUTHORIZED	11,070,000	11,070,000
PREFERRED SHARES ISSUED AND OUTSTANDING		10,589,434
REDEMPTION VALUE/ LIQUIDATION PREFERENCE		$110,450
CARRYING VALUE		110,246
COMMON STOCK ISSUABLE UPON CONVERSION		10,589,434
Series A Redeemable Convertible Preferred Stock [Member]
Temporary Equity [Line Items]
PREFERRED SHARES AUTHORIZED						880,000
PREFERRED SHARES ISSUED AND OUTSTANDING		855,996	335,999	489,999	19,999	9,999
REDEMPTION VALUE/ LIQUIDATION PREFERENCE		21,400
CARRYING VALUE		21,400
COMMON STOCK ISSUABLE UPON CONVERSION		855,996
Series B Redeemable Convertible Preferred Stock [Member]
Temporary Equity [Line Items]
PREFERRED SHARES AUTHORIZED									1,190,000
PREFERRED SHARES ISSUED AND OUTSTANDING		1,162,021					160,000	10,000	992,021
REDEMPTION VALUE/ LIQUIDATION PREFERENCE		29,050
CARRYING VALUE		29,050
COMMON STOCK ISSUABLE UPON CONVERSION		1,162,021
Series C Redeemable Convertible Preferred Stock [Member]
Temporary Equity [Line Items]
PREFERRED SHARES AUTHORIZED										9,000,000
PREFERRED SHARES ISSUED AND OUTSTANDING										8,571,417
REDEMPTION VALUE/ LIQUIDATION PREFERENCE										60,000
CARRYING VALUE										$59,796
COMMON STOCK ISSUABLE UPON CONVERSION										8,571,417

Common_Stock_and_Stock_Option_2

Common Stock and Stock Option Plan - Additional Information (Detail) (USD $)	0 Months Ended				12 Months Ended			1 Months Ended		12 Months Ended
In Thousands, except Share data, unless otherwise specified	Jul. 30, 2013	Feb. 09, 2012	Oct. 14, 2010	5-May-10	Dec. 31, 2014	Dec. 31, 2013	Dec. 31, 2012	Jul. 31, 2013	Jun. 30, 2011	Dec. 31, 2007	Sep. 24, 2013
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Voting rights by common stock holders					One vote
Stock option plan, additional stock authorized	1,715,851	4,800	45,214	30,254
Stock option plan, stock authorized	1,764,865	49,014
Stock option plan, vested rate					25.00%
Stock option plan, vesting period					36 months
Stock option plan, expiration period					10 years
Weighted-average grant date fair value of stock options granted					$9.62	$2.52	$30
Unrecognized compensation cost related to stock option grants					$18,284
Intrinsic value of stock options exercised					3,241	0	7
Cash received from stock option exercises					824
Stock options, shares outstanding					3,604,713	1,610,139					24,811
Stock options, weighted average exercise price					$11.28	$3.42					$45.16
Stock options, weighted average exercise price											$3.42
Stock option modification, incremental fair value						30
Stock option plan, reserved shares of common stock for future issuance					61,694
Stock options, granted					2,311,306
Stock-based compensation expense (credit)					8,237	495	125
Unrecognized compensation cost related to restricted common stock					498	0
Stock Options Granted to Nonemployees [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Unrecognized compensation cost related to stock option grants, recognition period					30 years
Unvested stock options					26,250
2014 Stock Option Plan [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Stock option plan, stock authorized					1,900,000
Stock option plan, vested rate					25.00%
Stock option plan, vesting period					36 months
Stock option plan, expiration period					10 years
Stock option plan, percentage of outstanding common stock					4.00%
Employee Stock Option
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Unrecognized compensation cost related to stock option grants, recognition period					3 years
Nonemployee Stock Option
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Stock options, granted					0	132,500	0
Stock options, granted fair value					337
Stock-based compensation expense (credit)					1,887	23	-19
Restricted Stock [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Restricted common stock, shares issued					44,000				200	320
Restricted common stock, fair value of shares issued					709				10	8
Remaining restricted common stock subject to repurchase					0	0
Restricted common stock, stock-based compensation expense					211	0	3
Restricted common stock, price per share							$50	$50	$50
Common stock subject to repurchase							125
Unvested common stock, repurchased								100
Total fair value of restricted stock awards vested					$57	$1	$3

Common_Stock_and_Stock_Option_3

Common Stock and Stock Option Plan - Assumptions for Estimating Fair Value of Options Granted Using Black-Scholes Option Pricing Model (Detail) (Employee Stock Option)	12 Months Ended
	Dec. 31, 2012	Dec. 31, 2014	Dec. 31, 2013
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Expected option term (in years)	6 years
Expected volatility	64.00%
Risk-free interest rate	0.70%
Expected dividend yield	0.00%
Minimum [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Expected option term (in years)		5 years 6 months
Expected volatility		64.00%	64.00%
Risk-free interest rate		1.70%	1.20%
Maximum [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Expected option term (in years)		6 years 3 months 18 days	6 years
Expected volatility		65.00%	69.00%
Risk-free interest rate		2.00%	1.90%
Expected dividend yield		0.00%	0.00%

Common_Stock_and_Stock_Option_4

Common Stock and Stock Option Plan - Summary of Stock Option Activity for Employee and Nonemployee Awards (Detail) (USD $)	12 Months Ended
	Dec. 31, 2014	Dec. 31, 2013	Sep. 24, 2013
NUMBER OF OPTIONS
Outstanding at beginning of year	1,610,139		24,811
Granted	2,311,306
Exercised	-239,853
Forfeited	-76,879
Outstanding at end of year	3,604,713	1,610,139	24,811
Exercisable at end of year	787,661
Vested and expected to vest at end of year	3,603,566
WEIGHTED- AVERAGE PRICE PER SHARE
Outstanding at beginning of year	$3.42		$45.16
Granted	$15.96
Exercised	$3.44
Forfeited	$11.72
Outstanding at end of year	$11.28	$3.42	$45.16
Exercisable at end of year	$7.52
Vested and expected to vest at end of year	$11.28
WEIGHTED- AVERAGE REMAINING CONTRACTUAL TERM (YEARS)
Outstanding at beginning of year	9 years 1 month 6 days	9 years 8 months 12 days
Exercisable at end of year	8 years 9 months 18 days
Vested and expected to vest at end of year	9 years 1 month 6 days

Common_Stock_and_Stock_Option_5

Common Stock and Stock Option Plan - Schedule of Valuation Assumptions (Detail) (Stock Options Granted to Employees [Member], USD $)	12 Months Ended
	Dec. 31, 2013	Dec. 31, 2012	Dec. 31, 2014
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Stock price	$7.42	$0.06
Expected option term (in years)	7 years	7 years
Expected volatility	66.00%	66.00%
Risk-free interest rate	2.80%	1.70%
Expected dividend yield	0.00%	0.00%
Minimum [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Stock price			$9.37
Expected option term (in years)			3 months
Expected volatility			56.00%
Risk-free interest rate			0.10%
Maximum [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Stock price			$41.12
Expected option term (in years)			6 years 10 months 10 days
Expected volatility			68.00%
Risk-free interest rate			2.30%
Expected dividend yield			0.00%

Common_Stock_and_Stock_Option_6

Common Stock and Stock Option Plan - Summary of Restricted Common Stock Activity (Detail) (USD $)	12 Months Ended
	Dec. 31, 2014	Dec. 31, 2014
Disclosure of Compensation Related Costs, Share-based Payments [Abstract]
Nonvested balance at beginning of year		40,000
Issued	44,000
Vested	-4,000
WEIGHTED- AVERAGE GRANT DATE FAIR VALUE PER SHARE
Issued	$16.30
Vested	$14.34
Nonvested balance at end of year	$16.30

Common_Stock_and_Stock_Option_7

Common Stock and Stock Option Plan - Classification of Stock-Based Compensation Expense (Detail) (USD $)	12 Months Ended
In Thousands, unless otherwise specified	Dec. 31, 2014	Dec. 31, 2013	Dec. 31, 2012
Employee Service Share-based Compensation, Allocation of Recognized Period Costs [Line Items]
Stock-based compensation	$8,237	$495	$125
Research and Development
Employee Service Share-based Compensation, Allocation of Recognized Period Costs [Line Items]
Stock-based compensation	4,183	124	14
General and Administrative
Employee Service Share-based Compensation, Allocation of Recognized Period Costs [Line Items]
Stock-based compensation	$4,054	$371	$111

Fair_Value_Measurements_Additi

Fair Value Measurements - Additional Information (Detail) (USD $)	12 Months Ended
	Dec. 31, 2014	Dec. 31, 2013
Debt Instrument Fair Value Carrying Value [Abstract]
Debt outstanding	$0
Preferred stock conversion date	4-Feb-14
Preferred stock, warrants outstanding	0
Transfers between Level 1 and Level 2	$0	$0

Fair_Value_Measurements_Schedu

Fair Value Measurements - Schedule of Roll-Forward of Company's Liabilities Measured at Fair Value on Recurring Basis Using Unobservable Inputs (Level 3) (Detail) (USD $)	12 Months Ended
In Thousands, unless otherwise specified	Dec. 31, 2014	Dec. 31, 2013	Dec. 31, 2012
Fair Value, Liabilities Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Balance at the beginning of the period	$529	$331	$800
Issuance of preferred stock warrants		324
Change in fair value of warrant liability	2,559	-126	-469
Balance at the end of the period		529	331
Level 3 [Member]
Fair Value, Liabilities Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Balance at the beginning of the period	529
Change in fair value of warrant liability	2,559
Transfers to Level 2	($3,088)

Fair_Value_Measurements_Schedu1

Fair Value Measurements - Schedule of Assets Measured or Disclosed at Fair Value on Recurring Basis (Detail) (USD $)	Dec. 31, 2014	Dec. 31, 2013
In Thousands, unless otherwise specified	Dec. 31, 2014	Dec. 31, 2013
Fair Value Assets And Liabilities Measured On Recurring Basis [Line Items]
Cash and cash equivalents	$20,425	$46,041
Held-to-maturity investments
U.S. treasury securities and government agency bonds	72,532
Assets held in restriction
Total	94,361	51,681
Long-term debt		4,847
Certificates of Deposit [Member]
Assets held in restriction
Certificates of deposit	1,380	264
U.S. Government Treasury and Agency Securities [Member]
Held-to-maturity investments
U.S. treasury securities and government agency bonds	72,532
Preferred Stock Warrant Liability [Member]
Assets held in restriction
Preferred stock warrant liability		529
Level 1[Member]
Fair Value Assets And Liabilities Measured On Recurring Basis [Line Items]
Cash and cash equivalents	20,425	46,041
Assets held in restriction
Total	20,425	46,041
Level 2 [Member]
Assets held in restriction
Total	73,936	5,111
Long-term debt		4,847
Level 2 [Member] \| Certificates of Deposit [Member]
Assets held in restriction
Certificates of deposit	1,380	264
Level 2 [Member] \| U.S. Government Treasury and Agency Securities [Member]
Held-to-maturity investments
U.S. treasury securities and government agency bonds	72,532
Level 3 [Member]
Assets held in restriction
Total		529
Level 3 [Member] \| Preferred Stock Warrant Liability [Member]
Assets held in restriction
Preferred stock warrant liability		$529

Income_Taxes_Additional_Inform

Income Taxes - Additional Information (Detail) (USD $)	12 Months Ended
Income Taxes - Additional Information (Detail) (USD $)	Dec. 31, 2014	Dec. 31, 2013	Dec. 31, 2012
Income Tax [Line Items]
Current income tax expense	$125,000	$0
Deferred income tax expense	125,000	0
Valuation allowance increased	18,732,000	8,116,000
Unrecognized tax benefit	1,216,000	865,000	563,000
Unrecognized tax benefits that would affect income tax expense if recognized	1,216,000
Federal [Member]
Income Tax [Line Items]
Deferred income tax expense	0	0
Net operating loss carryforwards	57,800,000
State [Member]
Income Tax [Line Items]
Net operating loss carryforwards	53,358,000
Research and development credits
Income Tax [Line Items]
Tax Credit Carryforward expiration Description	Expire starting in 2028.
Research and development credits \| Federal [Member]
Income Tax [Line Items]
Research and development credits	1,457,000
Research and development credits \| MASSACHUSETTS
Income Tax [Line Items]
Research and development credits	$902,000

Income_Taxes_Reconciliation_Be

Income Taxes - Reconciliation Between Income Taxes Computed at the Federal Statutory Income Tax Rate and the Provision for Benefit from Income Taxes (Detail)	12 Months Ended
	Dec. 31, 2014	Dec. 31, 2013	Dec. 31, 2012
Income Tax Disclosure [Abstract]
Federal statutory rate	34.00%	34.00%	34.00%
Effect of:
Change in valuation allowance	-32.00%	-35.40%	-33.20%
Research and development tax credit	0.80%	2.50%	-1.90%
Stock-based compensation	-2.60%	-1.30%	-0.40%
Other	-0.20%	0.20%	1.50%
Total	0.00%	0.00%	0.00%

Income_Taxes_Deferred_Tax_Asse

Income Taxes - Deferred Tax Assets (Detail) (USD $)	Dec. 31, 2014	Dec. 31, 2013
In Thousands, unless otherwise specified	Dec. 31, 2014	Dec. 31, 2013
Deferred tax assets
Net operating loss carryforwards	$41,846	$27,314
Capitalized research and development costs	2,646	1,465
Research and development credit carryforwards	2,360	1,806
Stock Compensation	2,479
Depreciation and other costs	372	386
Net deferred tax assets	49,703	30,971
Valuation allowance	-49,703	-30,971
Net deferred tax assets	$0	$0

Income_Taxes_Reconciliation_of

Income Taxes - Reconciliation of the Gross Unrecognized Tax Benefit (Detail) (USD $)	12 Months Ended
In Thousands, unless otherwise specified	Dec. 31, 2014	Dec. 31, 2013
Income Tax Disclosure [Abstract]
Unrecognized tax benefits at the beginning of the period	$865	$563
Additions for current tax positions	220	113
Changes for previous tax positions	131	189
Unrecognized tax benefits at the end of the period	$1,216	$865

Commitments_and_Contingencies_1

Commitments and Contingencies - Additional Information (Detail) (USD $)	12 Months Ended			0 Months Ended
	Dec. 31, 2014	Dec. 31, 2013	Dec. 31, 2012	Jul. 11, 2014	Dec. 30, 2014	Dec. 01, 2014
Loss Contingencies [Line Items]
Deferred rent obligation	$77,000	$105,000
Operating lease future minimum payments					3,259,000
Rent expense	741,000	875,000	899,000
Research and development expense	29,453,000	11,558,000	11,565,000
Facility Lease, 2008 Operating Lease Agreement [Member]
Loss Contingencies [Line Items]
Deferred rent obligation	67,000	105,000
Letter of credit	264,000
Lease Expiration Date	30-Nov-16
Cambridge Massachusetts , 2014 Operating Lease Agreement [Member]
Loss Contingencies [Line Items]
Deferred rent obligation	10,000
Letter of credit						1,116,000
Operating lease future minimum payments				9,498,000
Operating lease future minimum payments term				6 years
Deferred rent obligation				1-Dec-14
Plant Bioscience Limited License Agreement [Member]
Loss Contingencies [Line Items]
License agreement date	30-Sep-13
Payments on commencement of clinical trial	100,000
City of Hope License Agreement [Member]
Loss Contingencies [Line Items]
License agreement date	30-Sep-07
Research and development expense	50,000	500,000	1,500,000
Sub-license and other fees	0	0
Termination of license agreement description	The Company may terminate the license agreement at any time upon 90 days written notice
City of Hope License Agreement [Member] \| Minimum [Member]
Loss Contingencies [Line Items]
License fee			5,000,000
City of Hope License Agreement [Member] \| Maximum [Member]
Loss Contingencies [Line Items]
License fee			10,000,000
Tekmira [Member]
Loss Contingencies [Line Items]
License fee	3,000,000
Aggregate development milestones and royalty	$22,000,000

Commitments_and_Contingencies_2

Commitments and Contingencies - Summary of Future Minimum Payments on Lease Agreements (Detail) (USD $)	Dec. 30, 2014
In Thousands, unless otherwise specified	Dec. 30, 2014
Commitments and Contingencies Disclosure [Abstract]
2015	$2,101
2016	2,120
2017	1,582
2018	1,629
2019 and beyond	3,259
Total	$10,691

Commitments_and_Contingencies_3

Commitments and Contingencies - Summary of Future Minimum Payments on Lease Agreements (Parenthetical) (Detail) (USD $)	Dec. 30, 2014
In Thousands, unless otherwise specified	Dec. 30, 2014
Commitments and Contingencies Disclosure [Abstract]
Future minimum lease payments payable	$10,691
Associated offsetting sublease, gross	1,194
Associated offsetting sublease, 2015	610
Associated offsetting sublease, 2016	$584

Employee_Benefit_Plan_Addition

Employee Benefit Plan - Additional Information (Detail) (USD $)	12 Months Ended
In Thousands, unless otherwise specified	Dec. 31, 2014	Dec. 31, 2013	Dec. 31, 2011
Disclosure of Compensation Related Costs, Share-based Payments [Abstract]
Discretionary plan contribution	$175	$105	$127

Quarterly_Financial_Data_Detai

Quarterly Financial Data (Detail) (USD $)	3 Months Ended								12 Months Ended
In Thousands, except Per Share data, unless otherwise specified	Dec. 31, 2014	Sep. 30, 2014	Jun. 30, 2014	Mar. 31, 2014	Dec. 31, 2013	Sep. 30, 2013	Jun. 30, 2013	Mar. 31, 2013	Dec. 31, 2014	Dec. 31, 2013	Dec. 31, 2012
Quarterly Financial Information Disclosure [Abstract]
Revenue	$0	$0	$0	$0	$0	$0	$0	$0	$0	$0	$7,015
Net loss	-14,587	-11,193	-11,355	-10,804	-6,719	-4,194	-3,786	-3,819	-47,939	-18,518	-10,121
Net loss attributable to common stockholders	($14,587)	($11,193)	($11,355)	($11,008)	($6,728)	($4,540)	($4,808)	($4,830)	($48,143)	($20,906)	($14,218)
Net loss per share attributable to common stockholders-basic and diluted	($0.82)	($0.63)	($0.64)	($1.02)	($199.91)	($161.55)	($171.13)	($172.80)	($3)	($709.57)	($516)