QUARTERLY SUPPLEMENTARY FINANCIAL INFORMATION (UNAUDITED) (Details) - USD ($) $ / shares in Units, $ in Thousands	3 Months Ended								12 Months Ended
	Dec. 31, 2016	Sep. 30, 2016	Jun. 30, 2016	Mar. 31, 2016	Dec. 31, 2015	Sep. 30, 2015	Jun. 30, 2015	Mar. 31, 2015	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014
Quarterly Financial Data [Abstract]
Total revenues	$ 301,995	$ 289,494	$ 279,773	$ 265,685	$ 275,956	$ 264,560	$ 255,918	$ 238,484	$ 1,136,947	$ 1,034,918	$ 941,894
Income before income taxes	13,854	15,175	864	28,774	43,263	26,824	19,533	31,831	58,667	121,451	76,463
Net income	$ 11,995	$ 12,368	$ 3,114	$ 22,427	$ 30,895	$ 19,534	$ 15,018	$ 23,679	$ 49,904	$ 89,126	$ 59,137
Basic earnings per share (in usd per share)	$ 0.47	$ 0.49	$ 0.12	$ 0.89	$ 1.23	$ 0.78	$ 0.60	$ 0.95	$ 1.97	$ 3.56	$ 2.34
Diluted earnings per share (in usd per share)	$ 0.46	$ 0.48	$ 0.12	$ 0.88	$ 1.21	$ 0.77	$ 0.59	$ 0.94	$ 1.93	$ 3.53	$ 2.32