Income Taxes - Reconciliation of Income Tax Rate (Detail) - USD ($) $ in Thousands	3 Months Ended								12 Months Ended
	Dec. 31, 2019	Sep. 30, 2019	Jun. 30, 2019	Mar. 31, 2019	Dec. 31, 2018	Sep. 30, 2018	Jun. 30, 2018	Mar. 31, 2018	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
Effective Income Tax Rate Reconciliation, Amount
Tax at federal statutory rate									$ (6,499)	$ (26,936)	$ (52,089)
State tax, net of federal tax benefit									(60)	100	42
Stock-based compensation expense									4,773	6,559	3,171
Research and development tax credits									(2,336)	(7,839)	(5,022)
Change in valuation allowance									(802)	19,140	(3,532)
Change in unrecognized tax benefit									1,168	3,920	2,922
Tax rate change									0	0	53,048
Non-deductible expenses									3,250	5,143	2,212
Other									305	(44)	(120)
Income tax expense (benefit)	$ 140	$ 97	$ (438)	$ 0	$ 18	$ (38)	$ 24	$ 39	$ (201)	$ 43	$ 632