Earnings Per Share (Details) (USD $) In Thousands, unless otherwise specified	3 Months Ended		9 Months Ended
	Sep. 30, 2012	Sep. 30, 2011	Sep. 30, 2012	Sep. 30, 2011
Earnings Per Share, Basic and Diluted
Net earnings	$ 107,062	$ 60,437	$ 283,139	$ 176,905
Less: net earnings attributable to unvested restricted stock	(1,912)	(950)	(5,150)	(2,551)
Net earnings available to common stock	$ 105,150	$ 59,487	$ 277,989	$ 174,354
Weighted-average common shares outstanding	99,424	106,919	100,340	110,665
Dilutive effect of outstanding options (determined using treasury stock method)	276	129	261	245
Weighted-average common shares and potential common shares outstanding	99,700	107,048	100,601	110,910
Anti-dilutive stock options not included in treasury stock method computation	461	2,744	717	2,279