EARNINGS PER SHARE (Details) (USD $) In Thousands, except Per Share data, unless otherwise specified	3 Months Ended								12 Months Ended
	Dec. 31, 2011	Sep. 30, 2011	Jun. 30, 2011	Mar. 31, 2011	Dec. 31, 2010	Sep. 30, 2010	Jun. 30, 2010	Mar. 31, 2010	Dec. 31, 2011	Dec. 31, 2010	Dec. 31, 2009
Income (Numerator)
Income available to common shareholders (in dollars)	$ 38,620	$ 49,297	$ 51,289	$ 44,477	$ 40,781	$ 46,983	$ 46,695	$ 39,022	$ 183,683	$ 173,481	$ 124,623
Shares (Denominator)
Income available to common stockholders - Basic (in shares)	65,976	66,381	66,939	66,926	66,965	67,213	67,630	67,576	66,553	67,344	67,643
Income available to common stockholders - Diluted (in shares)	68,159	68,677	69,438	69,914	69,397	69,374	69,682	70,077	69,274	69,815	69,785
Per Share Amount
Income available to common stockholders - Basic (in dollars per share)	$ 0.59	$ 0.74	$ 0.77	$ 0.66	$ 0.61	$ 0.7	$ 0.69	$ 0.58	$ 2.76	$ 2.58	$ 1.84
Income available to common stockholders - Diluted (in dollars per share)	$ 0.57	$ 0.72	$ 0.74	$ 0.64	$ 0.59	$ 0.68	$ 0.67	$ 0.56	$ 2.65	$ 2.48	$ 1.79
Stock options
Shares (Denominator)
Effect of Dilutive Securities (in shares)									2,714	2,458	2,136
Restricted stock units
Shares (Denominator)
Effect of Dilutive Securities (in shares)									7	13	6