Condensed summary of significant accounting policies - Additional Information (Detail) (USD $)	3 Months Ended
	Mar. 31, 2014 Segment	Dec. 31, 2013	Mar. 31, 2013	Dec. 31, 2012
Significant Accounting Policies [Line Items]
Tax benefits realized upon ultimate settlement, description	A measurement attribute that measures a given tax position as the largest amount of tax benefits that are more than 50% likely of being realized upon ultimate settlement.
Percentage of tax benefit upon settlement	50.00%
Operating segment	1
Cash and cash equivalents	$ 52,918,327	$ 11,006,392	$ 19,017,252	$ 2,000,539
United States [Member]
Significant Accounting Policies [Line Items]
Cash and cash equivalents	$ 51,116,586	$ 6,629,383