Income Taxes - Reconciliation of Income Tax Rate (Detail) - USD ($) $ in Thousands	3 Months Ended								12 Months Ended
	Dec. 31, 2017	Sep. 30, 2017	Jun. 30, 2017	Mar. 31, 2017	Dec. 31, 2016	Sep. 30, 2016	Jun. 30, 2016	Mar. 31, 2016	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Effective Income Tax Rate Reconciliation, Amount
Tax at federal statutory rate									$ (52,089)	$ (51,072)	$ (738)
State tax, net of federal tax benefit									42	(1,028)	1,277
Stock-based compensation expense									3,171	3,509	549
Research and development tax credits									(5,022)	(688)	(1,068)
Change in valuation allowance									(3,532)	42,714	2,686
Change in unrecognized tax benefit									2,922	2,817	(62)
Tax rate change									53,048	0	0
Other									2,092	(480)	189
Income tax expense (benefit)	$ 711	$ 13	$ (52)	$ (40)	$ (224)	$ (209)	$ (3,946)	$ 151	$ 632	$ (4,228)	$ 2,833