Schedule II - Valuation and Qualifying Accounts (Details) - USD ($)		12 Months Ended
		Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Valuation and Qualifying Accounts [Roll Forward]
Balance at beginning of period		$ 3,443,296	$ 3,694,080	$ 4,748,657
Charged to costs and expenses		1,015,970	1,195,500	595,500
Charged to other accounts		20,400	19,500	25,500
Net accounts written off	[1]	(1,304,721)	(1,465,784)	(1,675,577)
Balance at end of period		$ 3,174,945	$ 3,443,296	$ 3,694,080

[1] Net of recovery of amounts previously written off.