EARNINGS PER SHARE (Details) - USD ($) $ / shares in Units, shares in Thousands, $ in Thousands	3 Months Ended																12 Months Ended
	Dec. 31, 2015		Sep. 30, 2015		Jun. 30, 2015		Mar. 31, 2015		Dec. 31, 2014		Sep. 30, 2014		Jun. 30, 2014		Mar. 31, 2014		Dec. 31, 2015		Dec. 31, 2014		Dec. 31, 2013
Numerator for common earnings per share:
Net income	$ 27,638		$ 22,873		$ 19,231		$ 14,714		$ 18,423		$ 19,040		$ 14,814		$ 17,364		$ 84,456		$ 69,641		$ 61,994
Numerator for basic and diluted earnings per share																	$ 84,456		$ 69,641		$ 61,994
Denominator for basic earnings per share:
Weighted average common shares outstanding (in shares)																	48,721		47,189		45,754
Effect of dilutive securities:
Stock options, RSUs and performance-based awards (in shares)																	3,265		2,545		2,604
Denominator for diluted earnings per share (in shares)																	51,986		49,734		48,358
Net income per share:
Basic (in dollars per share)	$ 0.56	[1]	$ 0.47	[1]	$ 0.40	[1]	$ 0.31	[1]	$ 0.39	[2]	$ 0.40	[2]	$ 0.31	[2]	$ 0.37	[2]	$ 1.73	[1]	$ 1.48	[2]	$ 1.35
Diluted (in dollars per share)	$ 0.52	[1]	$ 0.44	[1]	$ 0.37	[1]	$ 0.29	[1]	$ 0.37	[2]	$ 0.38	[2]	$ 0.30	[2]	$ 0.35	[2]	$ 1.62	[1]	$ 1.40	[2]	$ 1.28
Anti-dilutive options not included in the calculation (in shares)																	1,637		2,260		1,080

[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.