Condensed summary of significant accounting policies - Additional Information (Detail) (USD $)	9 Months Ended
	Sep. 30, 2014 Segment	Dec. 31, 2013
Significant Accounting Policies [Line Items]
Operating segment	1
Total Assets	$ 48,615,368	$ 15,649,106
Accumulated deficit	(81,325,064)	(65,693,708)
United States [Member]
Significant Accounting Policies [Line Items]
Total Assets	$ 42,113,885	$ 6,629,383