EARNINGS PER SHARE (Earning per share) (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
Numerator for basic and diluted earnings per share:
Net income	$ 59,965		$ 65,618		$ 50,255		$ 64,418		$ (30,874)		$ 42,634		$ 37,359		$ 23,641		$ 240,256		$ 72,760		$ 99,266
Numerator for basic and diluted earnings per share																	$ 240,256		$ 72,760		$ 99,266
Denominator:
Basic weighted average common stock outstanding (in shares)																	53,623,000		52,077,000		50,309,000
Stock options, equity-settled RSUs and performance-based awards (in shares)									0								3,050,000		2,907,000		2,906,000
Diluted weighted average common stock outstanding (in shares)																	56,673,000		54,984,000		53,215,000
Net Income per share:
Basic (in dollars per share)	$ 1.11	[1]	$ 1.22	[1]	$ 0.94	[1]	$ 1.21	[1]	$ (0.58)	[1],[2]	$ 0.81	[1]	$ 0.72	[1]	$ 0.46	[1]	$ 4.48	[1]	$ 1.40	[1]	$ 1.97
Diluted (in dollars per share)	$ 1.05	[1]	$ 1.15	[1]	$ 0.89	[1]	$ 1.15	[1]	$ (0.58)	[1],[2]	$ 0.77	[1]	$ 0.68	[1]	$ 0.44	[1]	$ 4.24	[1]	$ 1.32	[1]	$ 1.87
Anti-dilutive stock excluded from the calculation (in shares)																	139,000		883,000		2,325,000

[1] (1)Earnings per share amounts for each quarter may not necessarily total to the yearly earnings per share due to the weighting of shares outstanding on a quarterly and year to date basis.

[2] (2)Due to the net loss during the three months ended December 31, 2017, zero incremental shares are included in the calculation of diluted loss per share because of their antidilutive effect.