Schedule II - Valuation and qualifying accounts (Details) (USD $)	12 Months Ended
	Dec. 31, 2012	Dec. 31, 2011	Dec. 31, 2010
Allowance for doubtful accounts
Activity in valuation and qualifying accounts
Balance at beginning of period	$ 1,415,638	$ 1,354,694	$ 1,891,397
Additions charged to expenses	65,623	60,944
Deductions			(536,703)
Other adjustments	2,826
Balance at end of period	1,484,087	1,415,638	1,354,694
Deferred tax valuation allowance
Activity in valuation and qualifying accounts
Balance at beginning of period	26,452,590	26,326,256	25,102,856
Additions charged to expenses	2,845,531	1,218,415	1,618,532
Deductions	(59,857)	(1,092,081)	(395,132)
Balance at end of period	29,238,264	26,452,590	26,326,256
Tax valuation allowance
Valuation allowance related to depreciation and amortization	1,000,000
Valuation allowance related to net operating losses	1,100,000	900,000
Valuation allowance related to stock options and salaries and bonuses	700,000	300,000
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