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Stock-Based Compensation - Stock-Based Compensation Expense (Detail) (USD $)
 3 Months Ended
Mar. 31, 2014
Mar. 31, 2013
Employee Service Share-based Compensation, Allocation of Recognized Period Costs [Line Items]    
Total stock-based compensation expense and effect on net loss $ 521,106 $ 686,582
Effect on basic and diluted net loss per share $ (0.01) $ (0.02)
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 186,293 343,711
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 $ 334,813 $ 342,871