QUARTERLY SUPPLEMENTARY FINANCIAL INFORMATION (UNAUDITED) (Details) - USD ($) $ / shares in Units, $ in Thousands	3 Months Ended								12 Months Ended
	Dec. 31, 2015	Sep. 30, 2015	Jun. 30, 2015	Mar. 31, 2015	Dec. 31, 2014	Sep. 30, 2014	Jun. 30, 2014	Mar. 31, 2014	Dec. 31, 2015	Dec. 31, 2014	Dec. 31, 2013
Quarterly Financial Data [Abstract]
Total revenues	$ 275,956	$ 264,560	$ 255,918	$ 238,484	$ 250,452	$ 235,865	$ 232,673	$ 222,904	$ 1,034,918	$ 941,894	$ 877,042
Income before income taxes	43,263	26,824	19,533	31,831	50,355	(2,845)	13,056	15,897	121,451	76,463	101,902
Net income	$ 30,895	$ 19,534	$ 15,018	$ 23,679	$ 34,796	$ 325	$ 10,685	$ 13,331	$ 89,126	$ 59,137	$ 76,140
Basic earnings per share (in usd per share)	$ 1.23	$ 0.78	$ 0.60	$ 0.95	$ 1.39	$ 0.01	$ 0.42	$ 0.53	$ 3.56	$ 2.34	$ 3.01
Diluted earnings per share (in usd per share)	$ 1.21	$ 0.77	$ 0.59	$ 0.94	$ 1.38	$ 0.01	$ 0.42	$ 0.52	$ 3.53	$ 2.32	$ 2.98