EARNINGS PER SHARE (Earning per share) (Details) - USD ($) $ / shares in Units, $ in Thousands	3 Months Ended																12 Months Ended
	Dec. 31, 2019		Sep. 30, 2019		Jun. 30, 2019		Mar. 31, 2019		Dec. 31, 2018		Sep. 30, 2018		Jun. 30, 2018		Mar. 31, 2018		Dec. 31, 2019		Dec. 31, 2018		Dec. 31, 2017
Numerator for basic and diluted earnings per share:
Net income	$ 74,524		$ 67,002		$ 58,777		$ 60,754		$ 59,965		$ 65,618		$ 50,255		$ 64,418		$ 261,057		$ 240,256		$ 72,760
Numerator for basic and diluted earnings per share																	$ 261,057		$ 240,256		$ 72,760
Denominator:
Basic weighted average common stock outstanding (in shares)																	54,719,414		53,622,989		52,077,011
Stock options, equity-settled RSUs and performance-based awards (in shares)																	2,948,375		3,049,687		2,907,162
Diluted weighted average common stock outstanding (in shares)																	57,667,789		56,672,676		54,984,173
Net Income per share:
Basic (in dollars per share)	$ 1.35	[1]	$ 1.22	[1]	$ 1.07	[1]	$ 1.12	[1]	$ 1.11	[2]	$ 1.22	[2]	$ 0.94	[2]	$ 1.21	[2]	$ 4.77	[1]	$ 4.48	[2]	$ 1.40
Diluted (in dollars per share)	$ 1.29	[1]	$ 1.16	[1]	$ 1.02	[1]	$ 1.06	[1]	$ 1.05	[2]	$ 1.15	[2]	$ 0.89	[2]	$ 1.15	[2]	$ 4.53	[1]	$ 4.24	[2]	$ 1.32
Anti-dilutive stock excluded from the calculation (in shares)																	120,021		138,639		883,350

[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] (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.