UNAUDITED INTERIM FINANCIAL INFORMATION (Details) - USD ($) $ / shares in Units, $ in Thousands	3 Months Ended																12 Months Ended
	Dec. 31, 2017		Sep. 30, 2017		Jun. 30, 2017		Mar. 31, 2017		Dec. 31, 2016		Sep. 30, 2016		Jun. 30, 2016		Mar. 31, 2016		Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Quarterly Financial Information Disclosure [Abstract]
Total revenues	$ 174,320		$ 165,918		$ 164,150		$ 154,457		$ 168,398		$ 162,378		$ 160,336		$ 153,871		$ 658,845	$ 644,983	$ 623,726
Income (loss) before income tax expense (benefit)	24,587		(440)		7,229		8,440		29,196		5,047		8,087		20,877		39,816	63,207	71,656
Income tax expense (benefit)	(1,597)		(1,186)		1,725		1,636		7,904		918		1,959		6,223		578	17,004	21,494
Net income	$ 26,184		$ 746		$ 5,504		$ 6,804		$ 21,292		$ 4,129		$ 6,128		$ 14,654		$ 39,238	$ 46,203	$ 50,162
Net income per common share - basic and diluted1	$ 1.23	[1]	$ 0.03	[1]	$ 0.26	[1]	$ 0.32	[1]	$ 1.01	[1]	$ 0.20	[1]	$ 0.29	[1]	$ 0.70	[1]	$ 1.84	$ 2.20	$ 2.43

[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 per share may not total to annual net income per share.