Intangible assets (Details) (USD $)	9 Months Ended
	Sep. 30, 2012	Sep. 30, 2011	Dec. 31, 2011
Intangible assets [Abstract]
Intangible assets	$ 25,664,015		$ 21,429,488
Accumulated amortization	(4,574,354)		(2,809,972)
Intangible assets, net	21,089,661		18,619,516
Intangible assets, useful life	10 years
Amortization of intangible asset	$ 1,764,382	$ 1,626,476