Summary of Significant Accounting Policies - Summary of Impact of Adoption to Consolidated Statement of Income and Balance Sheet (Detail) - USD ($) $ / shares in Units, $ in Thousands	3 Months Ended
	Mar. 31, 2018	Mar. 31, 2017	Jan. 01, 2018	Dec. 31, 2017
Revenue Initial Application Period Cumulative Effect Transition [Line Items]
Collaboration revenue	$ 7,469	$ 6,215
Operating loss	(22,430)	(12,948)
Net loss	$ (21,356)	$ (12,631)
Net loss per share, basic and diluted	$ (0.51)	$ (0.36)
Liabilities:
Deferred revenue - current	$ 17,062			$ 21,188
Deferred revenue - noncurrent	38,332			44,111
Stockholders' Equity:
Accumulated deficit	(137,038)			$ (121,113)
Accounting Standards Update (“ASU”) No. 2014-09 [Member]
Liabilities:
Deferred revenue - current			$ 18,419
Deferred revenue - noncurrent			41,449
Stockholders' Equity:
Accumulated deficit			(115,682)
Accounting Standards Update (“ASU”) No. 2014-09 [Member] | Balances without Adoption of ASC 606 [Member]
Revenue Initial Application Period Cumulative Effect Transition [Line Items]
Collaboration revenue	7,674
Operating loss	(22,225)
Net loss	$ (21,151)
Net loss per share, basic and diluted	$ (0.50)
Liabilities:
Deferred revenue - current	$ 20,404
Deferred revenue - noncurrent	40,216
Stockholders' Equity:
Accumulated deficit	(142,264)
Accounting Standards Update (“ASU”) No. 2014-09 [Member] | Effect of Change Higher/(Lower) [Member]
Revenue Initial Application Period Cumulative Effect Transition [Line Items]
Collaboration revenue	(205)
Operating loss	(205)
Net loss	$ (205)
Net loss per share, basic and diluted	$ (0.01)
Liabilities:
Deferred revenue - current	$ (3,342)		(2,769)
Deferred revenue - noncurrent	(1,884)		(2,662)
Stockholders' Equity:
Accumulated deficit	$ 5,226		$ 5,431