Schedule II - Valuation and qualifying accounts (Details) (USD $)	12 Months Ended
	Dec. 31, 2013	Dec. 31, 2012	Dec. 31, 2011
Tax valuation allowance
Trade receivable written-off	$ 1,400,000
Allowance for doubtful accounts
Activity in valuation and qualifying accounts
Balance at beginning of period	1,484,087	1,415,638	1,354,694
Additions charged to expenses	326,457	65,623	60,944
Deductions	(1,640,870)
Other adjustments		2,826
Balance at end of period	169,674	1,484,087	1,415,638
Tax valuation allowance
Trade receivable written-off	1,300,000
Deferred tax valuation allowance
Activity in valuation and qualifying accounts
Balance at beginning of period	29,238,264	26,452,590	26,326,256
Additions charged to expenses	903,463	2,845,531	1,218,415
Deductions	(725,263)	(59,857)	(1,092,081)
Balance at end of period	29,416,464	29,238,264	26,452,590
Tax valuation allowance
Valuation allowance related to stock options and restricted stock	200,000
Increase in allowance attributable to decrease in bonus	200,000
Increase in allowance attributable to decrease in bad debt	500,000
Valuation allowance related to net operating losses	400,000	1,100,000	900,000
Valuation allowance related to stock options and salaries and bonuses		700,000	300,000
Valuation allowance related to depreciation and amortization		1,000,000
Decrease in allowance attributable to increase in deferred tax liability related to depreciation and amortization			900,000
Decrease in allowance attributable to reduction in allowance for doubtful debts			$ 200,000