EARNINGS PER SHARE (Details) (USD $) In Thousands, except Per Share data, unless otherwise specified	3 Months Ended								12 Months Ended
	Dec. 31, 2012	Sep. 30, 2012	Jun. 30, 2012	Mar. 31, 2012	Dec. 31, 2011	Sep. 30, 2011	Jun. 30, 2011	Mar. 31, 2011	Dec. 31, 2012	Dec. 31, 2011	Dec. 31, 2010
Income (Numerator)
Income available to common shareholders (in dollars)	$ 34,990	$ 42,127	$ 41,686	$ 43,809	$ 38,620	$ 49,297	$ 51,289	$ 44,477	$ 162,612	$ 183,683	$ 173,481
Shares (Denominator)
Income available to common stockholders - Basic (in shares)	66,249	66,541	66,580	66,196	65,976	66,381	66,939	66,926	66,392	66,553	67,344
Income available to common stockholders - Diluted (in shares)	67,846	68,353	68,758	68,785	68,159	68,677	69,438	69,914	68,395	69,274	69,815
Per Share Amount
Income available to common stockholders - Basic (in dollars per share)	$ 0.53	$ 0.63	$ 0.63	$ 0.66	$ 0.59	$ 0.74	$ 0.77	$ 0.66	$ 2.45	$ 2.76	$ 2.58
Income available to common stockholders - Diluted (in dollars per share)	$ 0.52	$ 0.62	$ 0.61	$ 0.64	$ 0.57	$ 0.72	$ 0.74	$ 0.64	$ 2.38	$ 2.65	$ 2.48
Stock options
Shares (Denominator)
Effect of Dilutive Securities (in shares)									1,990	2,714	2,458
Restricted stock units
Shares (Denominator)
Effect of Dilutive Securities (in shares)									13	7	13