EARNINGS PER SHARE (Earning per share) (Details) - USD ($) $ / shares in Units, shares in Thousands, $ in Thousands	3 Months Ended																12 Months Ended
	Dec. 31, 2016		Sep. 30, 2016		Jun. 30, 2016		Mar. 31, 2016		Dec. 31, 2015		Sep. 30, 2015		Jun. 30, 2015		Mar. 31, 2015		Dec. 31, 2016		Dec. 31, 2015		Dec. 31, 2014
Numerator for common earnings per share:
Net income	$ 24,763		$ 26,174		$ 24,430		$ 23,899		$ 27,638		$ 22,873		$ 19,231		$ 14,714		$ 99,266		$ 84,456		$ 69,641
Numerator for basic and diluted earnings per share																	$ 99,266		$ 84,456		$ 69,641
Denominator for basic and diluted earnings per share:
Weighted average common shares outstanding (in shares)																	50,309		48,721		47,189
Effect of dilutive securities:
Stock options, RSUs and performance-based awards (in shares)																	2,906		3,265		2,545
Denominator for diluted earnings per share (in shares)																	53,215		51,986		49,734
Net income per share:
Basic (in dollars per share)	$ 0.49	[1]	$ 0.51	[1]	$ 0.49	[1]	$ 0.48	[1]	$ 0.56	[1]	$ 0.47	[1]	$ 0.40	[1]	$ 0.31	[1]	$ 1.97	[1]	$ 1.73	[1]	$ 1.48
Diluted (in dollars per share)	$ 0.46	[1]	$ 0.49	[1]	$ 0.46	[1]	$ 0.45	[1]	$ 0.52	[1]	$ 0.44	[1]	$ 0.37	[1]	$ 0.29	[1]	$ 1.87	[1]	$ 1.62	[1]	$ 1.40
Anti-dilutive options not included in the calculation (in shares)																	2,325		1,637		2,260

[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.