Summary of Significant Accounting Policies - Basic and Diluted Net Loss per Share Computations (Detail) (USD $)	3 Months Ended		6 Months Ended
	Jun. 30, 2014	Jun. 30, 2013	Jun. 30, 2014	Jun. 30, 2013
Accounting Policies [Abstract]
Net loss	$ (12,115,224)	$ (5,868,682)	$ (19,735,480)	$ (12,285,129)
Weighted average shares outstanding used to compute basic and diluted net income or loss per share	55,711,717	39,661,934	55,529,818	38,966,547
Basic and diluted net loss per share	$ (0.22)	$ (0.15)	$ (0.36)	$ (0.32)