Long and short-term capital leases (Details) (USD $)	12 Months Ended
	Dec. 31, 2014	Dec. 31, 2013	Dec. 31, 2012
Long and short-term capital leases
Less: short-term portion capital leases	$ (458,812)	$ (466,010)
Capital leases to various lessors secured by financed equipment with interest rates ranging from [4.67% to 14.51%]
Long and short-term capital leases
Total long and short-term portions of capital leases	816,718	949,596
Less: short-term portion capital leases	(458,812)	(466,010)
Capital Lease Obligations, Noncurrent	357,906	483,586
Total term debt interest expense	$ 61,591	$ 60,106	$ 55,699
Capital leases to various lessors secured by financed equipment with interest rates ranging from [4.67% to 14.51%] | Minimum
Long and short-term capital leases
Interest rate (as a percent)	4.67%	4.67%
Capital leases to various lessors secured by financed equipment with interest rates ranging from [4.67% to 14.51%] | Maximum
Long and short-term capital leases
Interest rate (as a percent)	14.51%	14.51%