Income Taxes - Effect of Temporary Differences and Carryforwards (Detail) - USD ($) $ in Thousands	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014	Dec. 31, 2013
Income Tax Disclosure [Abstract]
Net operating loss carryforwards	$ 70,694	$ 57,158
Research and development tax credit carryforwards	4,572	3,124
Stock-based compensation	5,360	3,212
Other	7,827	7,135
Total deferred tax assets	88,453	70,629
Less: Valuation allowance	(88,453)	(70,629)	$ (53,923)	$ (36,012)
Net deferred tax assets	$ 0	$ 0