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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
Dec. 31, 2011
Assets:      
Cash and cash equivalents $ 7,522,029 $ 18,695,197 $ 31,049,050
Money market funds 26,047,169 29,047,958 24,169,311
Restricted cash 404,850 404,850 582,171
Total assets 33,974,048 48,148,005 55,800,532
Liabilities:      
Preferred stock warrant liability (679,296) (52,947) (203,642)
Quoted Prices in Active Markets for Identical Assets Level 1 [Member]
      
Assets:      
Cash and cash equivalents 7,522,029 18,695,197 31,049,050
Money market funds 26,047,169 29,047,958  
Restricted cash 404,850 404,850 582,171
Total assets 33,974,048 48,148,005 31,631,221
Liabilities:      
Preferred stock warrant liability         
Significant Other Observable Inputs Level 2 [Member]
      
Assets:      
Cash and cash equivalents         
Money market funds       24,169,311
Restricted cash         
Total assets       24,169,311
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) $ (203,642)