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
Income (Numerator)
Income available to common stockholders (in dollars)	$ 41,598	$ 48,595	$ 53,076	$ 48,389	$ 36,899	$ 45,264	$ 49,802	$ 40,029	$ 191,658	$ 171,994	$ 162,612
Shares (Denominator)
Basic (in shares)	64,368	64,886	65,328	65,468	65,700	66,092	66,420	66,155	65,009	66,090	66,392
Total average equivalent shares	66,121	66,845	67,438	68,232	67,853	67,986	68,106	68,296	67,292	68,208	68,395
Per Share Amount
Net income per share, basic (in dollars per share)	$ 0.65	$ 0.75	$ 0.81	$ 0.74	$ 0.56	$ 0.68	$ 0.75	$ 0.61	$ 2.95	$ 2.60	$ 2.45
Net income per share, diluted (in dollars per share)	$ 0.63	$ 0.73	$ 0.79	$ 0.71	$ 0.54	$ 0.67	$ 0.73	$ 0.59	$ 2.85	$ 2.52	$ 2.38
Stock options
Shares (Denominator)
Effect of dilutive stock based compensation (in shares)									2,248	2,106	1,990
Restricted stock units
Shares (Denominator)
Effect of dilutive stock based compensation (in shares)									35	12	13