SUMMARY OF SIGNIFICANT ACCOUNTING POLICIES (Details 7) - 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
Earnings per share:
Net income (loss)									$ 67,181	$ 48,675	$ 45,017
Weighted average common shares, basic (in shares)									48,009,000	47,953,000	47,865,000
Dilutive SARs (treasury method) (in shares)									0	3,000	15,000
Weighted average common shares, dilutive (in shares)									48,009,000	47,956,000	47,880,000
Earnings per share—basic (in dollars per share)									$ 1.40	$ 1.02	$ 0.94
Earnings per share—diluted (in dollars per share)	$ 0.29	$ 0.70	$ 0.39	$ 0.02	$ 0.31	$ 0.48	$ 0.24	$ (0.02)	$ 1.40	$ 1.01	$ 0.94
Stock Appreciation Rights (SARs)
Stock-based awards included in earnings per share
Stock Appreciation Rights (SAR) outstanding (in shares)	0				0				0	0	64,500