Collaboration, Research Grant and License Agreements - Additional Information (Detail) - USD ($)			3 Months Ended								12 Months Ended
	Jan. 31, 2014	Jan. 03, 2014	Dec. 31, 2016	Sep. 30, 2016	Jun. 30, 2016	Mar. 31, 2016	Dec. 31, 2015	Sep. 30, 2015	Jun. 30, 2015	Mar. 31, 2015	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014	Dec. 31, 2013	May 31, 2013
Collaborative Arrangements and Non-collaborative Arrangement Transactions [Line Items]
Revenue			$ 4,060,000	$ 1,715,000	$ 1,451,000	$ 1,158,000	$ 1,234,000	$ 1,201,000	$ 1,070,000	$ 807,000	$ 8,384,000	$ 4,312,000	$ 5,150,000
Research and development			10,461,000	$ 9,218,000	$ 7,404,000	$ 6,876,000	5,787,000	$ 6,258,000	$ 5,976,000	$ 4,503,000	$ 33,959,000	22,524,000	16,744,000
Astellas Collaboration Agreement [Member]
Collaborative Arrangements and Non-collaborative Arrangement Transactions [Line Items]
Convertible promissory notes													5,000,000
Initial research term											3 years 6 months
Proceeds from collaborator											$ 2,500,000	3,600,000
Description deliverables under agreement											(i) the research license, (ii) the research services to be provided over the research term, which is three and a half years, and (iii) the Company’s participation in the Joint Research Committee (the “Committee”) to be provided over the initial three and a half year research term of the agreement.
Earned non-substantive milestone payments											$ 1,800,000	800,000
Revenue											3,200,000	1,300,000	0
Deferred revenue			3,000,000				3,200,000				3,000,000	3,200,000
Astellas Collaboration Agreement [Member] | Maximum [Member]
Collaborative Arrangements and Non-collaborative Arrangement Transactions [Line Items]
Development and sales milestone payments											$ 398,500,000
Biogen License Agreements [Member]
Collaborative Arrangements and Non-collaborative Arrangement Transactions [Line Items]
Proceeds from collaborator		$ 2,500,000
Description deliverables under agreement											(i) the research license, (ii) the research services to be provided over the four-year research term of the agreement, and (iii) the Company’s participation in the Joint Research Committee (the “Committee”) to be provided over the initial four-year research term of the agreement.
Earned non-substantive milestone payments													2,000,000
Revenue											$ 5,200,000	3,000,000	2,400,000
Deferred revenue			0				5,100,000				$ 0	5,100,000	5,100,000
Purchase of preferred stock under existing terms purchased		$ 5,000,000
Research funding payments guaranteed period		2 years
Research funding payments		$ 4,000,000
Agreement termination date											Dec. 06, 2016
CFFT [Member]
Collaborative Arrangements and Non-collaborative Arrangement Transactions [Line Items]
Revenue											$ 0	0	2,600,000
Funds received for research, development and commercialization agreement	$ 5,700,000
Description of funds received in different periods											Under terms of the CFFT Agreement, which was subsequently amended in May 2013 and January 2014, CFFT agreed to provide up to $5.7 million (the “Award’) in research funding to the Company over two non-consecutive one-year periods from March 2012 to March 2013 and from January 2014 to December 2014.
Milestone and royalty payments description											(a) 268,265 shares of the Company’s common stock if the change of control occurs prior to the selection by the Company of a compound intended for product approval, or (b) 444,025 shares of the Company’s common stock if the change of control occurs after the selection by the Company of a compound intended for product approval. This alternative payment option upon a change of control would be cash settled in the event of a change of control and meets the definition of an embedded derivative.
Estimated fair value of derivative liability			100,000								$ 100,000
Payment obligations			$ 0								$ 0
Agreement termination period description											Each party may terminate for an uncured material breach of any material covenants or obligations or if any representation or warranty is materially untrue as of the date made and uncured after 30 days from notice. CFFT may also terminate if a case or proceeding under the bankruptcy laws is filed against the Company and not dismissed within 60 days, or if the Company files for insolvency, reorganization, receivership, dissolution or liquidation.
CFFT [Member] | Occurs Prior to Selection [Member]
Collaborative Arrangements and Non-collaborative Arrangement Transactions [Line Items]
Common stock			268,265								268,265
CFFT [Member] | Occurs After Selection [Member]
Collaborative Arrangements and Non-collaborative Arrangement Transactions [Line Items]
Common stock			444,025								444,025
CFFT [Member] | Maximum [Member]
Collaborative Arrangements and Non-collaborative Arrangement Transactions [Line Items]
Payments for Royalties											$ 34,200,000
Royalty Expense											22,800,000
Estimated fair value of derivative liability							$ 100,000					100,000
MJFF Research Grant [Member]
Collaborative Arrangements and Non-collaborative Arrangement Transactions [Line Items]
Revenue											0	0	200,000
Funds received from research grants															$ 500,000
Harvard Agreement [Member]
Collaborative Arrangements and Non-collaborative Arrangement Transactions [Line Items]
Research and development											$ 100,000	$ 100,000	$ 100,000
Harvard Agreement [Member] | Maximum [Member]
Collaborative Arrangements and Non-collaborative Arrangement Transactions [Line Items]
Future payment for clinical development milestones														$ 3,500,000
Future payments for commercial milestone														6,000,000
Increase in clinical development milestone payments under licensing agreement														15,400,000
Increase in commercial milestone payments under licensing agreement														103,500,000
Sublicense Agreement [Member] | Maximum [Member]
Collaborative Arrangements and Non-collaborative Arrangement Transactions [Line Items]
Milestone payment obligation if sublicense rights are granted														$ 1,000,000