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Fair Value Measurements - Additional Information (Detail) (USD $)
12 Months Ended 3 Months Ended 12 Months Ended
Dec. 31, 2012
Sep. 30, 2013
Aug. 31, 2013
Sep. 30, 2013
Series C warrant [Member]
Sep. 30, 2013
Preferred and common stock warrants [Member]
Sep. 30, 2013
Convertible notes payable [Member]
Dec. 31, 2012
March 2012 and October 2012 convertible notes [Member]
Dec. 31, 2012
Subordinated convertible notes [Member]
Sep. 30, 2013
Subordinated convertible notes [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]                  
Fair value discount rate 35.00%           25.00% 18.00%  
Discount for lack of marketability 18.00%                
Warrants liability reclassified   $ 2,669,000              
Public offering price of the shares sold     $ 12.00 $ 12.00         $ 12
Transfers from Level 3 to Level 2         $ 574,000 $ 48,234,000