Income Taxes - Deferred Tax Assets (Detail) - USD ($) $ in Thousands	Dec. 31, 2016	Dec. 31, 2015
Income Tax Disclosure [Abstract]
Net operating loss carryforwards	$ 85,148	$ 83,558
Stock based compensation	435	333
Research and development credits	2,925	2,628
Fixed assets	2,617	1
Accruals	391	239
Total Deferred tax assets	91,516	86,759
Valuation allowance	(91,516)	(86,759)
Net deferred tax asset	$ 0	$ 0