Stock-Based Compensation Expense (Detail) - USD ($) $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2015	Sep. 30, 2014	Sep. 30, 2015	Sep. 30, 2014
Employee Service Share Based Compensation Allocation Of Recognized Period Costs [Line Items]
Non-cash stock-based compensation	$ 1,255	$ 859	$ 1,953	$ 2,362
Stock options and employee stock purchase plan awards | Research and Development Expense
Employee Service Share Based Compensation Allocation Of Recognized Period Costs [Line Items]
Non-cash stock-based compensation	29	96	103	355
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]
Non-cash stock-based compensation	104	225	279	764
Unvested restricted common stock | Research and Development Expense
Employee Service Share Based Compensation Allocation Of Recognized Period Costs [Line Items]
Non-cash stock-based compensation	255	90	436	361
Unvested restricted common stock | Selling, General and Administrative Expense
Employee Service Share Based Compensation Allocation Of Recognized Period Costs [Line Items]
Non-cash stock-based compensation	838	$ 448	1,106	$ 882
Restricted stock units | Research and Development Expense
Employee Service Share Based Compensation Allocation Of Recognized Period Costs [Line Items]
Non-cash stock-based compensation	4		4
Restricted stock units | Selling, General and Administrative Expense
Employee Service Share Based Compensation Allocation Of Recognized Period Costs [Line Items]
Non-cash stock-based compensation	$ 25		$ 25