Intangible assets (Details) (USD $)	12 Months Ended
	Dec. 31, 2014	Dec. 31, 2013	Dec. 31, 2012
Intangible assets, net [Abstract]
Intangible assets	$ 52,562,549	$ 54,719,918
Accumulated amortization	(13,714,153)	(8,511,833)
Intangible assets, net	38,848,396	46,208,085
Intangible assets, useful life	10 years
Amortization of intangible assets	7,359,690	3,295,716	2,446,975
Adjustments to intangible assets due to deferred tax liability	2,157,369
Accumulated amortization of intangible assets	269,671
Amortization expense for intangible assets	1,887,698
Intangible assets future amortization expense [Abstract]
2015	5,256,255
2016	5,256,255
2017	5,256,255
2018	5,256,255
2019	5,256,255
Thereafter	12,567,121
Intangible assets, net	$ 38,848,396	$ 46,208,085