Summary of Stock Based Compensation Expense Related to Employee Stock Options (Detail) (USD $) In Thousands, unless otherwise specified	3 Months Ended		6 Months Ended
	Jun. 30, 2013	Jun. 30, 2012	Jun. 30, 2013	Jun. 30, 2012
Employee Service Share-based Compensation, Allocation of Recognized Period Costs [Line Items]
Non-cash charges for stock-based compensation	$ 305	$ 308	$ 626	$ 638
Research and development
Employee Service Share-based Compensation, Allocation of Recognized Period Costs [Line Items]
Non-cash charges for stock-based compensation	201	205	420	428
General and administrative
Employee Service Share-based Compensation, Allocation of Recognized Period Costs [Line Items]
Non-cash charges for stock-based compensation	$ 104	$ 103	$ 206	$ 210