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Summary of Significant Accounting Policies - Basic and Dilutive Net Loss per Share Computations (Detail) (USD $)
 3 Months Ended 12 Months Ended
Dec. 31, 2013
Sep. 30, 2013
Jun. 30, 2013
Mar. 31, 2013
Dec. 31, 2012
Sep. 30, 2012
Jun. 30, 2012
Mar. 31, 2012
Dec. 31, 2013
Dec. 31, 2012
Dec. 31, 2011
Accounting Policies [Abstract]                      
Net loss $ (6,962,000) $ (7,192,000) $ (5,869,000) $ (6,416,000) $ (2,758,000) $ (16,337,000) $ 833,000 $ (10,229,000) $ (26,439,159) $ (28,491,153) $ (21,328,538)
Weighted average shares outstanding used to compute basic and diluted net loss per share                 43,422,001 28,824,417 14,187,885
Basic and diluted net loss per share $ (0.13) $ (0.17) $ (0.15) $ (0.17) $ (0.07) $ (0.54) $ 0.03 $ (0.45) $ (0.61) $ (0.99) $ (1.50)