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Stock-Based Compensation - Stock-Based Compensation Expense (Detail) (USD $)
 12 Months Ended
Dec. 31, 2013
Dec. 31, 2012
Dec. 31, 2011
Employee Service Share-based Compensation, Allocation of Recognized Period Costs [Line Items]      
Total stock-based compensation expense and effect on net loss $ 2,608,670 $ 2,878,134 $ 3,260,937
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 1,219,308 1,280,298 1,680,431
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 $ 1,389,362 $ 1,597,836 $ 1,580,506