EARNINGS PER SHARE (Details) (USD $) In Thousands, except Per Share data, unless otherwise specified	3 Months Ended								12 Months Ended
	Dec. 31, 2014	Sep. 30, 2014	Jun. 30, 2014	Mar. 31, 2014	Dec. 31, 2013	Sep. 30, 2013	Jun. 30, 2013	Mar. 31, 2013	Dec. 31, 2014	Dec. 31, 2013	Dec. 31, 2012
Numerator for common earnings per share:
Net income	$ 18,423	$ 19,040	$ 14,814	$ 17,364	$ 18,759	$ 16,437	$ 14,118	$ 12,680	$ 69,641	$ 61,994	$ 54,484
Net income allocated to participating securities									0	0	(3,341)
Numerator for basic earnings per share									69,641	61,994	51,143
Effect on income available from reallocation of options									0	0	261
Numerator for diluted earnings per share									$ 69,641	$ 61,994	$ 51,404
Denominator for basic earnings per share:
Weighted average common shares outstanding (in shares)									47,189	45,754	40,190
Effect of dilutive securities:
Stock options, RSUs and performance-based awards (in shares)									2,545	2,604	3,631
Denominator for diluted earnings per share (in shares)									49,734	48,358	43,821
Net income per share:
Basic (in dollars per share)	$ 0.39	$ 0.40	$ 0.31	$ 0.37	$ 0.40	$ 0.36	$ 0.31	$ 0.28	$ 1.48	$ 1.35	$ 1.27
Diluted (in dollars per share)	$ 0.37	$ 0.38	$ 0.30	$ 0.35	$ 0.38	$ 0.34	$ 0.29	$ 0.27	$ 1.40	$ 1.28	$ 1.17
Anti-dilutive options not included in the calculation (in shares)									2,260	1,080	1,534