Stock-Based Compensation - Stock-Based Compensation Expense (Detail) (USD $)	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]
Total stock-based compensation	$ 749,641	$ 723,445	$ 1,436,223	$ 1,459,990
Effect on basic and diluted net loss per share	$ (0.02)	$ (0.03)	$ (0.04)	$ (0.06)
Research and development expense [Member]
Employee Service Share-based Compensation, Allocation of Recognized Period Costs [Line Items]
Total stock-based compensation	373,652	320,303	717,363	652,177
Selling, general and administrative expense [Member]
Employee Service Share-based Compensation, Allocation of Recognized Period Costs [Line Items]
Total stock-based compensation	$ 375,989	$ 403,142	$ 718,860	$ 807,813