Cash and cash equivalents - Schedule of cash and cash equivalents (Detail) (USD $)	Mar. 31, 2014	Dec. 31, 2013	Mar. 31, 2013	Dec. 31, 2012
Cash And Cash Equivalents [Abstract]
Cash	$ 49,538,874	$ 7,292,116
Cash equivalents	3,379,453	3,714,276
Cash and cash equivalents	$ 52,918,327	$ 11,006,392	$ 19,017,252	$ 2,000,539