Organization and Summary of Significant Accounting Policies - Consolidated Statement of Operations and Comprehensive Loss (Detail) - USD ($) $ / shares in Units, $ in Thousands	3 Months Ended								12 Months Ended
	Dec. 31, 2018	Sep. 30, 2018	Jun. 30, 2018	Mar. 31, 2018	Dec. 31, 2017	Sep. 30, 2017	Jun. 30, 2017	Mar. 31, 2017	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Revenue Initial Application Period Cumulative Effect Transition [Line Items]
Revenues	$ 26,837	$ 23,562	$ 21,416	$ 12,637	$ 13,077	$ 11,812	$ 8,253	$ 3,425	$ 84,452	$ 36,567	$ 19,389
Net loss	$ (18,664)	$ (12,843)	$ (16,640)	$ (20,187)	$ (13,091)	$ (12,354)	$ (12,491)	$ (16,632)	$ (68,334)	$ (54,568)	$ (71,658)
Net loss per share - basic and diluted:	$ (0.18)	$ (0.13)	$ (0.17)	$ (0.23)	$ (0.15)	$ (0.15)	$ (0.17)	$ (0.23)	$ (0.70)	$ (0.70)	$ (1.02)
Adjustments [Member] | Topic 606 [Member]
Revenue Initial Application Period Cumulative Effect Transition [Line Items]
Net loss									$ (13,558)
Net loss per share - basic and diluted:									$ (0.14)
Balances without adoption of Topic 606 [Member] | Topic 606 [Member]
Revenue Initial Application Period Cumulative Effect Transition [Line Items]
Net loss									$ (81,892)
Net loss per share - basic and diluted:									$ (0.84)
Collaboration Agreements [Member]
Revenue Initial Application Period Cumulative Effect Transition [Line Items]
Revenues									$ 84,065	$ 35,960	$ 18,881
Collaboration Agreements [Member] | Adjustments [Member] | Topic 606 [Member]
Revenue Initial Application Period Cumulative Effect Transition [Line Items]
Revenues									(13,558)
Collaboration Agreements [Member] | Balances without adoption of Topic 606 [Member] | Topic 606 [Member]
Revenue Initial Application Period Cumulative Effect Transition [Line Items]
Revenues									$ 70,507