Earnings Per Share (Details) (USD $) In Thousands, unless otherwise specified	3 Months Ended		9 Months Ended
	Sep. 30, 2013	Sep. 30, 2012	Sep. 30, 2013	Sep. 30, 2012
Earnings Per Share, Basic and Diluted
Net earnings	$ 98,175	$ 107,062	$ 292,187	$ 283,139
Less: net earnings attributable to unvested restricted stock	(1,527)	(1,912)	(4,754)	(5,150)
Net earnings available to common stock	$ 96,648	$ 105,150	$ 287,433	$ 277,989
Weighted-average common shares outstanding	98,723	99,424	98,881	100,340
Dilutive effect of outstanding securities (determined using treasury stock method)	270	276	254	261
Weighted-average common shares and potential common shares outstanding	98,993	99,700	99,135	100,601
Anti-dilutive securities not included in treasury stock method computation	131	461	89	717