Intangible assets (Details) (USD $)	3 Months Ended
	Mar. 31, 2013	Mar. 31, 2012	Dec. 31, 2012
Intangible assets, net [Abstract]
Intangible assets	$ 25,702,908		$ 25,702,908
Accumulated amortization	(5,858,689)		(5,216,116)
Intangible assets, net	19,844,219		20,486,792
Intangible assets, useful life	10 years
Amortization of intangible assets	$ 642,573	$ 535,737