Earnings Per Share (Details) (USD $) In Thousands, 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
Earnings Per Share, Basic and Diluted
Net earnings	$ 108,101	$ 107,062	$ 93,493	$ 82,584	$ 78,338	$ 60,437	$ 69,478	$ 46,990	$ 391,240	$ 255,243	$ 345,096
Less: net earnings attributable to unvested restricted stock									(6,982)	(3,864)	(3,926)
Net earnings available to common stock									$ 384,258	$ 251,379	$ 341,170
Weighted-average common shares outstanding	99,686	99,424	99,563	102,034	104,260	106,919	111,389	113,754	100,176	109,051	113,863
Dilutive effect of outstanding options (determined using treasury stock method)									280	189	214
Weighted-average common shares and potential common shares outstanding	99,926	99,700	99,851	102,193	104,356	107,048	111,757	114,106	100,456	109,240	114,077
Anti-dilutive stock options not included in treasury stock method computation									443	2,426	4,451