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Summary of Significant Accounting Policies - Summary of Fair Value Measurement Financial Asset and Liabilities (Detail) (USD $)
Sep. 30, 2013
Dec. 31, 2012
Assets:    
Cash and cash equivalents $ 7,522,029 $ 18,695,197
Money market funds 26,047,169 29,047,958
Restricted cash 404,850 404,850
Total assets 33,974,048 48,148,005
Liabilities:    
Preferred stock warrant liability (679,296) (52,947)
Quoted Prices in Active Markets for Identical Assets Level 1 [Member]
    
Assets:    
Cash and cash equivalents 7,522,029 18,695,197
Money market funds 26,047,169 29,047,958
Restricted cash 404,850 404,850
Total assets 33,974,048 48,148,005
Liabilities:    
Preferred stock warrant liability      
Significant Other Observable Inputs Level 2 [Member]
    
Assets:    
Cash and cash equivalents      
Money market funds      
Restricted cash      
Total assets      
Liabilities:    
Preferred stock warrant liability      
Significant Unobservable Inputs Level 3 [Member]
    
Assets:    
Cash and cash equivalents      
Money market funds      
Restricted cash      
Total assets      
Liabilities:    
Preferred stock warrant liability $ (679,296) $ (52,947)