Income Taxes - Components of Deferred Tax Assets (Detail) - USD ($) $ in Thousands	Dec. 31, 2017	Dec. 31, 2016
Income Tax Disclosure [Abstract]
Net operating loss carryforwards	$ 35,409	$ 44,929
Research and development credit carryforwards	7,301	6,017
Compensation expense	652	2,735
Other	1,467	1,266
Total deferred tax assets	44,829	54,947
Valuation allowance for deferred tax assets	$ (44,829)	(54,772)
Net deferred tax assets		$ 175