Stock-Based Compensation - Summary of Compensation Expense Recognized With Regard to Stock Options Granted to Non-employee Consultants Including Effect of Re-measurement of Fair Values of Options (Detail) - Non-employee Consultants [Member] - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Share-based Compensation Arrangement by Share-based Payment Award, Compensation Cost [Line Items]
Total stock option expense including effect of re-measurement of fair value	$ 370	$ (99)	$ 350
Research and Development [Member]
Share-based Compensation Arrangement by Share-based Payment Award, Compensation Cost [Line Items]
Total stock option expense including effect of re-measurement of fair value	170	(79)	250
General and Administrative [Member]
Share-based Compensation Arrangement by Share-based Payment Award, Compensation Cost [Line Items]
Total stock option expense including effect of re-measurement of fair value	$ 200	$ (20)	$ 100