Schedule II - Valuation and qualifying accounts (Details) - USD ($)	12 Months Ended
	Dec. 31, 2015	Dec. 31, 2014	Dec. 31, 2013
Allowance for doubtful accounts
Activity in valuation and qualifying accounts
Balance at beginning of period	$ 108,765	$ 169,674	$ 1,484,087
Additions charged to expenses	8,388	46,652	326,457
Deductions	(24,799)	(107,561)	(1,640,870)
Balance at end of period	92,354	108,765	169,674
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	28,475,080	29,416,464	29,238,264
Additions charged to expenses	653,947	843,479	903,463
Deductions	(8,567,511)	(1,784,863)	(725,263)
Balance at end of period	$ 20,561,516	$ 28,475,080	29,416,464
Tax valuation allowance
Valuation allowance related to net operating losses			400,000
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 stock options and salaries and bonuses			$ 200,000