Long and short-term capital leases (Details) (USD $)	12 Months Ended
	Dec. 31, 2013	Dec. 31, 2012	Dec. 31, 2011
Long and short-term capital leases
Less: short-term portion capital leases	$ (466,010)	$ (337,588)
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	949,596	701,598
Less: short-term portion capital leases	(466,010)	(337,588)
Long-term portion of capital leases	483,586	364,010
Total term debt interest expense	$ 60,106	$ 55,699	$ 64,462
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%