QUARTERLY SUPPLEMENTARY FINANCIAL INFORMATION (UNAUDITED) (Details) (USD $) In Thousands, except Per Share data, unless otherwise specified	3 Months Ended								12 Months Ended
	Dec. 31, 2014	Sep. 30, 2014	Jun. 30, 2014	Mar. 31, 2014	Dec. 31, 2013	Sep. 30, 2013	Jun. 30, 2013	Mar. 31, 2013	Dec. 31, 2014		Dec. 31, 2013		Dec. 31, 2012
Quarterly Financial Data [Abstract]
Total revenues	$ 250,452	$ 235,865	$ 232,673	$ 222,904	$ 228,994	$ 223,024	$ 219,719	$ 205,305	$ 941,894		$ 877,042		$ 813,243
Income (loss) before income taxes	50,355	(2,845)	13,056	15,897	37,339	14,395	20,318	29,850	76,463		101,902		46,074
Net income	$ 34,796	$ 325	$ 10,685	$ 13,331	$ 26,526	$ 11,725	$ 15,496	$ 22,393	$ 59,137		$ 76,140		$ 40,212
Basic earnings per share (in usd per share)	$ 1.39	$ 0.01	$ 0.42	$ 0.53	$ 1.04	$ 0.46	$ 0.61	$ 0.89	$ 2.34	[1]	$ 3.01	[1]	$ 1.58
Diluted earnings per share (in usd per share)	$ 1.38	$ 0.01	$ 0.42	$ 0.52	$ 1.04	$ 0.45	$ 0.61	$ 0.88	$ 2.32	[1]	$ 2.98	[1]	$ 1.58

[1] The sum of the quarterly reported amounts may not equal the full year, as each is computed independently.