Intangible assets (Details) (USD $)	12 Months Ended
	Dec. 31, 2012	Dec. 31, 2011	Dec. 31, 2010
Intangible assets, net [Abstract]
Intangible assets	$ 25,702,909	$ 21,429,488
Accumulated amortization	(5,216,117)	(2,809,972)
Intangible assets, net	20,486,792	18,619,516
Intangible assets, useful life	10 years
Amortization of intangible assets	2,446,975	1,991,200	790,117
Increase in accumulated amortization	40,830
Intangible assets future amortization expense [Abstract]
2013	2,570,291
2014	2,570,291
2015	2,570,291
2016	2,570,291
2017	2,570,291
Thereafter	7,635,337
Intangible assets, net	$ 20,486,792	$ 18,619,516