Fair Value Measurement - Financial Assets and Liabilities Measured at Fair Value (Detail) (USD $)	Jun. 30, 2013	Dec. 31, 2012
Financial assets:
Total financial assets	$ 1,699,503	$ 14,339,896
Financial liabilities:
Total financial liabilities	8,277,443
Fair Value, Measurements [Member]
Financial assets:
Total financial assets	23,927,887
Financial liabilities:
Total financial liabilities	18,094,881
Fair Value, Measurements [Member] \| Marketable securities [Member]
Financial assets:
Total financial assets	1,426,061	13,900,678
Fair Value, Measurements [Member] \| Money market funds [Member] \| Cash equivalents [Member]
Financial assets:
Total financial assets	521,269
Fair Value, Measurements [Member] \| U.S. Treasury debt obligations [Member] \| Cash equivalents [Member]
Financial assets:
Total financial assets	21,980,557
Fair Value, Measurements [Member] \| Debt securities [Member] \| Marketable securities [Member]
Financial assets:
Total financial assets	1,426,061
Fair Value, Measurements [Member] \| Bond obligation [Member]
Financial assets:
Total financial assets	231,250	331,250
Financial liabilities:
Total financial liabilities	231,250
Fair Value, Measurements [Member] \| Loan payable net of discounts [Member]
Financial liabilities:
Total financial liabilities	9,586,188
Fair Value, Measurements [Member] \| Warrant liabilities [Member]
Financial assets:
Total financial assets	42,192	107,968
Financial liabilities:
Total financial liabilities	8,277,443
Quoted Prices in Active Markets for Identical Assets (Level 1) [Member] \| Fair Value, Measurements [Member]
Financial assets:
Total financial assets	22,501,826
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	521,269
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	21,980,557
Quoted Prices in Active Markets for Identical Assets (Level 1) [Member] \| Fair Value, Measurements [Member] \| Debt securities [Member] \| Marketable securities [Member]
Financial assets:
Total financial assets
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	1,426,061
Financial liabilities:
Total financial liabilities	273,442
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] \| Debt securities [Member] \| Marketable securities [Member]
Financial assets:
Total financial assets	1,426,061
Significant Other Observable Inputs Level 2 [Member] \| Fair Value, Measurements [Member] \| Bond obligation [Member]
Financial liabilities:
Total financial liabilities	231,250
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 liabilities:
Total financial liabilities	42,192
UNobservable Inputs (Level 3) [Member] \| Fair Value, Measurements [Member]
Financial assets:
Total financial assets
Financial liabilities:
Total financial liabilities	17,821,439
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] \| Debt securities [Member] \| Marketable securities [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	9,586,188
UNobservable Inputs (Level 3) [Member] \| Fair Value, Measurements [Member] \| Warrant liabilities [Member]
Financial liabilities:
Total financial liabilities	$ 8,235,251