UNAUDITED INTERIM FINANCIAL INFORMATION (Details) (USD $)	3 Months Ended																12 Months Ended
	Dec. 31, 2013		Sep. 30, 2013		Jun. 30, 2013		Mar. 31, 2013		Dec. 31, 2012		Sep. 30, 2012		Jun. 30, 2012		Mar. 31, 2012		Dec. 31, 2013	Dec. 31, 2012	Dec. 31, 2011
Quarterly Financial Information Disclosure [Abstract]
Total revenues	$ 146,686,003		$ 144,109,477		$ 138,566,944		$ 133,842,713		$ 128,727,934		$ 132,562,158		$ 120,503,570		$ 130,073,865		$ 568,351,347	$ 511,867,527	$ 472,644,641
Income (loss) before income tax expense (benefit)	22,829,935		9,564,866		7,948,655		20,508,964		18,496,775		10,750,356		(5,511,804)		27,898,287		60,852,420	51,633,614	(10,991,517)
Income tax expense (benefit)	6,995,658		2,365,223		1,736,419		6,236,447		5,498,854		2,429,112		(2,935,333)		8,674,552		17,333,747	13,667,185	(8,254,238)
Net income (loss)	$ 15,834,277		$ 7,199,643		$ 6,212,236		$ 14,272,517		$ 12,997,921		$ 8,321,244		$ (2,576,471)		$ 19,223,735		$ 43,518,673	$ 37,966,429	$ (2,737,279)
Net income (loss) per common share - basic and diluted (1)	$ 1.20	[1]	$ 0.55	[1]	$ 0.48	[1]	$ 1.10	[1]	$ 1.01	[1]	$ 0.65	[1]	$ (0.20)	[1]	$ 1.49	[1]	$ 3.33	$ 2.95	$ (0.21)

[1] Since the weighted-average number of shares outstanding for the quarters are calculated independently of the weighted-average number of shares outstanding for the year, quarterly net income (loss) per share may not total to annual net income (loss) per share.