Condensed summary of significant accounting policies - Additional Information (Detail) (USD $)	6 Months Ended
	Jun. 30, 2014 Segment	Dec. 31, 2013
Significant Accounting Policies [Line Items]
Operating segment	1
Cash and cash equivalents	$ 52,963,752	$ 15,649,106
Accumulated deficit	(75,040,984)	(65,693,708)
United States [Member]
Significant Accounting Policies [Line Items]
Cash and cash equivalents	$ 46,104,742	$ 6,629,383