Stock-Based Compensation Expense (Detail) - USD ($) $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2016	Sep. 30, 2015	Sep. 30, 2016	Sep. 30, 2015
Employee Service Share Based Compensation Allocation Of Recognized Period Costs [Line Items]
Stock-based compensation	$ 270	$ 1,260	$ 812	$ 1,953
Stock options and employee stock purchase plan awards | Research and Development Expense
Employee Service Share Based Compensation Allocation Of Recognized Period Costs [Line Items]
Stock-based compensation	15	29	56	103
Stock options and employee stock purchase plan awards | Selling, General and Administrative Expense
Employee Service Share Based Compensation Allocation Of Recognized Period Costs [Line Items]
Stock-based compensation	94	103	294	279
Unvested restricted common stock | Research and Development Expense
Employee Service Share Based Compensation Allocation Of Recognized Period Costs [Line Items]
Stock-based compensation	20	255	101	436
Unvested restricted common stock | Selling, General and Administrative Expense
Employee Service Share Based Compensation Allocation Of Recognized Period Costs [Line Items]
Stock-based compensation	26	838	122	1,106
Restricted stock units | Research and Development Expense
Employee Service Share Based Compensation Allocation Of Recognized Period Costs [Line Items]
Stock-based compensation	13	4	27	4
Restricted stock units | Selling, General and Administrative Expense
Employee Service Share Based Compensation Allocation Of Recognized Period Costs [Line Items]
Stock-based compensation	$ 102	$ 31	$ 212	$ 25