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Shared-Based Payments - Schedule of Stock-Based Compensation Expense (Detail) (USD $)
 9 Months Ended 12 Months Ended
Sep. 30, 2013
Sep. 30, 2012
Dec. 31, 2012
Dec. 31, 2011
Employee Service Share-based Compensation, Allocation of Recognized Period Costs [Line Items]        
Stock-based compensation expense $ 393,561 $ 628,796 $ 838,395 $ 2,347,439
Research and Development Expense [Member]
        
Employee Service Share-based Compensation, Allocation of Recognized Period Costs [Line Items]        
Stock-based compensation expense 284,281 353,856 471,809 1,018,935
General and Administrative Expense [Member]
        
Employee Service Share-based Compensation, Allocation of Recognized Period Costs [Line Items]        
Stock-based compensation expense $ 109,280 $ 274,940 $ 366,586 $ 1,328,504