Fair Value Measurement - Financial Assets and Liabilities Measured at Fair Value (Detail) (USD $)	Sep. 30, 2013	Dec. 31, 2012
Warrant liabilities [Member]
Financial liabilities:
Total financial liabilities	$ 8,420,954
Fair Value, Measurements [Member]
Financial assets:
Total financial assets	20,955,293
Financial liabilities:
Total financial liabilities	22,068,052
Fair Value, Measurements [Member] \| Money market funds [Member] \| Cash equivalents [Member]
Financial assets:
Total financial assets	377,695
Fair Value, Measurements [Member] \| U.S. Treasury debt obligations [Member] \| Cash equivalents [Member]
Financial assets:
Total financial assets	20,577,598
Fair Value, Measurements [Member] \| Bond obligation [Member]
Financial liabilities:
Total financial liabilities	178,750
Fair Value, Measurements [Member] \| Loan payable net of discounts [Member]
Financial liabilities:
Total financial liabilities	13,468,348
Fair Value, Measurements [Member] \| Warrant liabilities [Member]
Financial liabilities:
Total financial liabilities	8,420,954
Quoted Prices in Active Markets for Identical Assets (Level 1) [Member] \| Fair Value, Measurements [Member]
Financial assets:
Total financial assets	20,955,293
Financial liabilities:
Total financial liabilities
Quoted Prices in Active Markets for Identical Assets (Level 1) [Member] \| Fair Value, Measurements [Member] \| Money market funds [Member] \| Cash equivalents [Member]
Financial assets:
Total financial assets	377,695
Quoted Prices in Active Markets for Identical Assets (Level 1) [Member] \| Fair Value, Measurements [Member] \| U.S. Treasury debt obligations [Member] \| Cash equivalents [Member]
Financial assets:
Total financial assets	20,577,598
Quoted Prices in Active Markets for Identical Assets (Level 1) [Member] \| Fair Value, Measurements [Member] \| Bond obligation [Member]
Financial liabilities:
Total financial liabilities
Quoted Prices in Active Markets for Identical Assets (Level 1) [Member] \| Fair Value, Measurements [Member] \| Loan payable net of discounts [Member]
Financial liabilities:
Total financial liabilities
Quoted Prices in Active Markets for Identical Assets (Level 1) [Member] \| Fair Value, Measurements [Member] \| Warrant liabilities [Member]
Financial liabilities:
Total financial liabilities
Significant Other Observable Inputs Level 2 [Member] \| Fair Value, Measurements [Member]
Financial assets:
Total financial assets
Financial liabilities:
Total financial liabilities	194,010
Significant Other Observable Inputs Level 2 [Member] \| Fair Value, Measurements [Member] \| Money market funds [Member] \| Cash equivalents [Member]
Financial assets:
Total financial assets
Significant Other Observable Inputs Level 2 [Member] \| Fair Value, Measurements [Member] \| U.S. Treasury debt obligations [Member] \| Cash equivalents [Member]
Financial assets:
Total financial assets
Significant Other Observable Inputs Level 2 [Member] \| Fair Value, Measurements [Member] \| Bond obligation [Member]
Financial assets:
Total financial assets	178,750	331,250
Financial liabilities:
Total financial liabilities	178,750
Significant Other Observable Inputs Level 2 [Member] \| Fair Value, Measurements [Member] \| Loan payable net of discounts [Member]
Financial liabilities:
Total financial liabilities
Significant Other Observable Inputs Level 2 [Member] \| Fair Value, Measurements [Member] \| Warrant liabilities [Member]
Financial assets:
Total financial assets	15,260	107,968
Financial liabilities:
Total financial liabilities	15,260
UNobservable Inputs (Level 3) [Member] \| Fair Value, Measurements [Member]
Financial assets:
Total financial assets
Financial liabilities:
Total financial liabilities	21,874,042
UNobservable Inputs (Level 3) [Member] \| Fair Value, Measurements [Member] \| Money market funds [Member] \| Cash equivalents [Member]
Financial assets:
Total financial assets
UNobservable Inputs (Level 3) [Member] \| Fair Value, Measurements [Member] \| U.S. Treasury debt obligations [Member] \| Cash equivalents [Member]
Financial assets:
Total financial assets
UNobservable Inputs (Level 3) [Member] \| Fair Value, Measurements [Member] \| Bond obligation [Member]
Financial liabilities:
Total financial liabilities
UNobservable Inputs (Level 3) [Member] \| Fair Value, Measurements [Member] \| Loan payable net of discounts [Member]
Financial liabilities:
Total financial liabilities	13,468,348
UNobservable Inputs (Level 3) [Member] \| Fair Value, Measurements [Member] \| Warrant liabilities [Member]
Financial liabilities:
Total financial liabilities	$ 8,405,694