Schedule II. - VALUATION AND QUALIFYING ACCOUNTS (Details) (Allowance for doubtful accounts, USD $)	12 Months Ended
	Dec. 31, 2012	Dec. 31, 2011	Dec. 31, 2010
Allowance for doubtful accounts
Valuation and Qualifying Accounts
Balance at beginning of year	$ 1,312,845	$ 2,450,715	$ 2,411,283
Charged to cost and expenses	(7,924)	84,798	740,387
Deduction	(576,199)	(1,222,668)	(700,955)
Balance at end of year	$ 728,722	$ 1,312,845	$ 2,450,715