Intangible assets (Details) (USD $)	12 Months Ended
	Dec. 31, 2013	Dec. 31, 2012	Dec. 31, 2011
Intangible assets, net [Abstract]
Intangible assets	$ 54,719,918	$ 25,702,909
Accumulated amortization	(8,511,833)	(5,216,117)
Intangible assets, net	46,208,085	20,486,792
Intangible assets, useful life	10 years
Amortization of intangible assets	3,295,716	2,446,975	1,991,200
Intangible assets future amortization expense [Abstract]
2014	5,472,123
2015	5,472,123
2016	5,472,123
2017	5,472,123
Thereafter	24,319,593
Intangible assets, net	$ 46,208,085	$ 20,486,792