EARNINGS PER SHARE (Earning per share) (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
Numerator for basic and diluted earnings per share:
Net income	$ (30,874)		$ 42,634		$ 37,359		$ 23,641		$ 24,763		$ 26,174		$ 24,430		$ 23,899		$ 72,760	$ 99,266	$ 84,456
Numerator for basic and diluted earnings per share																	$ 72,760	$ 99,266	$ 84,456
Denominator for basic and diluted earnings per share:
Basic weighted average common stock outstanding (in shares)																	52,077,011	50,309,362	48,720,844
Stock options, equity-settled RSUs and performance-based awards (in shares)	0																2,907,162	2,906,030	3,265,029
Diluted weighted average common stock outstanding (in shares)																	54,984,173	53,215,392	51,985,873
Earnings per share:
Basic (in dollars per share)	$ (0.58)	[1],[2]	$ 0.81	[1]	$ 0.72	[1]	$ 0.46	[1]	$ 0.49	[1]	$ 0.51	[1]	$ 0.49	[1]	$ 0.48	[1]	$ 1.40	$ 1.97	$ 1.73
Diluted (in dollars per share)	$ (0.58)	[1],[2]	$ 0.77	[1]	$ 0.68	[1]	$ 0.44	[1]	$ 0.46	[1]	$ 0.49	[1]	$ 0.46	[1]	$ 0.45	[1]	$ 1.32	$ 1.87	$ 1.62
Anti-dilutive stock excluded from the calculation (in shares)																	883,350	2,324,667	1,637,048

[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 earnings per share because of their antidilutive effect.