Stock Compensation Plans - Schedule of Stock-based Compensation Expense to Employees and Non-Employees (Detail) - 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]
Stock-based compensation expense	$ 5,480	$ 4,678	$ 4,029
Employees and Directors
Share-based Compensation Arrangement by Share-based Payment Award, Compensation Cost [Line Items]
Stock-based compensation expense	5,387	4,509	3,424
Non-Employees
Share-based Compensation Arrangement by Share-based Payment Award, Compensation Cost [Line Items]
Stock-based compensation expense	93	169	605
Research and Development | Employees and Directors
Share-based Compensation Arrangement by Share-based Payment Award, Compensation Cost [Line Items]
Stock-based compensation expense	1,645	1,758	1,412
Research and Development | Non-Employees
Share-based Compensation Arrangement by Share-based Payment Award, Compensation Cost [Line Items]
Stock-based compensation expense	93	153	490
General and Administrative | Employees and Directors
Share-based Compensation Arrangement by Share-based Payment Award, Compensation Cost [Line Items]
Stock-based compensation expense	3,742	2,751	2,012
General and Administrative | Non-Employees
Share-based Compensation Arrangement by Share-based Payment Award, Compensation Cost [Line Items]
Stock-based compensation expense	$ 0	$ 16	$ 115