Earnings Per Share (Details) (USD $) In Thousands, unless otherwise specified	3 Months Ended		6 Months Ended
	Jun. 30, 2013	Jun. 30, 2012	Jun. 30, 2013	Jun. 30, 2012
Earnings Per Share, Basic and Diluted
Net earnings	$ 88,162	$ 93,493	$ 194,012	$ 176,077
Less: net earnings attributable to unvested restricted stock	(1,445)	(1,775)	(3,227)	(3,238)
Net earnings available to common stock	$ 86,717	$ 91,718	$ 190,785	$ 172,839
Weighted-average common shares outstanding	98,870	99,563	98,965	100,802
Dilutive effect of outstanding options (determined using treasury stock method)	251	288	245	181
Weighted-average common shares and potential common shares outstanding	99,121	99,851	99,210	100,983
Anti-dilutive stock options not included in treasury stock method computation	109	642	89	1,990