Stock-Based Compensation - Stock-Based Compensation Expense (Detail) (USD $)	3 Months Ended		6 Months Ended
	Jun. 30, 2014	Jun. 30, 2013	Jun. 30, 2014	Jun. 30, 2013
Employee Service Share-based Compensation, Allocation of Recognized Period Costs [Line Items]
Total stock-based compensation expense and effect on net loss	$ 513,660	$ 749,641	$ 1,034,766	$ 1,436,223
Effect on basic and diluted net loss per share	$ (0.01)	$ (0.02)	$ (0.02)	$ (0.04)
Research and development expense [Member]
Employee Service Share-based Compensation, Allocation of Recognized Period Costs [Line Items]
Total stock-based compensation expense and effect on net loss	39,596	373,652	225,889	717,363
Selling, general and administrative expense [Member]
Employee Service Share-based Compensation, Allocation of Recognized Period Costs [Line Items]
Total stock-based compensation expense and effect on net loss	$ 474,064	$ 375,989	$ 808,877	$ 718,860