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Warrant Liability - Summary of Changes in Fair Value of Warrant Liability (Detail) (USD $)
 9 Months Ended
Sep. 30, 2013
Class of Warrant or Right [Line Items]  
Number of warrants, ending balance 9,601,378
Series A Warrants [Member]
  
Class of Warrant or Right [Line Items]  
Number of warrants, beginning balance 8,501,429
Number of warrants exercised (334,534)
Number of warrants, ending balance 8,166,895
Fair value of warrant liability, beginning balance $ 9,157,397
Fair value of warrants exercised (418,841)
Fair value, changes in fair value at period end (332,862)
Fair value of warrant liability, ending balance $ 8,405,694