UNAUDITED INTERIM FINANCIAL INFORMATION (Details) - USD ($) $ / shares in Units, $ in Thousands	3 Months Ended																12 Months Ended
	Dec. 31, 2018		Sep. 30, 2018		Jun. 30, 2018		Mar. 31, 2018		Dec. 31, 2017		Sep. 30, 2017		Jun. 30, 2017		Mar. 31, 2017		Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Quarterly Financial Information Disclosure [Abstract]
Total revenues	$ 141,586		$ 189,125		$ 166,637		$ 163,379		$ 175,598		$ 167,196		$ 165,426		$ 155,737		$ 660,727	$ 663,957	$ 649,792
Income (loss) before income tax expense (benefit)	(30,870)		24,444		(7,722)		(528)		24,587		(440)		7,229		8,440		(14,676)	39,816	63,207
Income tax expense (benefit)	(9,325)		5,296		(2,727)		(452)		(1,597)		(1,186)		1,725		1,636		(7,208)	578	17,004
Net income (loss)	$ (21,545)		$ 19,148		$ (4,995)		$ (76)		$ 26,184		$ 746		$ 5,504		$ 6,804		$ (7,468)	$ 39,238	$ 46,203
Net income (loss) per common share - basic and diluted1	$ (1.00)	[1]	$ 0.89	[1]	$ (0.24)	[1]	$ 0.00	[1]	$ 1.23	[1]	$ 0.03	[1]	$ 0.26	[1]	$ 0.32	[1]	$ (0.35)	$ 1.84	$ 2.20

[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.