Cash and cash equivalents - Schedule of Cash and Cash Equivalents (Detail) (USD $)	Jun. 30, 2014	Mar. 28, 2014	Dec. 31, 2013	Jun. 30, 2013	Mar. 31, 2013	Dec. 31, 2012
Cash And Cash Equivalents [Abstract]
Cash	$ 2,144,656		$ 7,292,116
Cash equivalents	15,751,454		3,714,276
Cash and cash equivalents	$ 17,896,110	$ 52,918,327	$ 11,006,392	$ 17,589,632	$ 19,017,252	$ 2,000,539