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	3 Months Ended
	Mar. 31, 2018	Mar. 31, 2017
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	$ 167	$ 183
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	29	144
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	$ 138	$ 39