Earnings Per Share (Details) (USD $) In Thousands, unless otherwise specified	3 Months Ended								12 Months Ended
	Dec. 31, 2011	Sep. 30, 2011	Jun. 30, 2011	Mar. 31, 2011	Dec. 31, 2010	Sep. 30, 2010	Jun. 30, 2010	Mar. 31, 2010	Dec. 31, 2011	Dec. 31, 2010	Dec. 31, 2009
Earnings Per Share, Basic and Diluted
Net earnings	$ 78,338	$ 60,437	$ 69,478	$ 46,990	$ 97,299	$ 93,063	$ 83,380	$ 71,354	$ 255,243	$ 345,096	$ 353,868
Less: net earnings attributable to unvested restricted stock and restricted stock units									(3,864)	(3,926)	(1,928)
Net earnings available to common stock									$ 251,379	$ 341,170	$ 351,940
Weighted-average common shares outstanding	104,260	106,919	111,389	113,754	113,834	114,002	113,935	113,668	109,051	113,863	112,200
Dilutive effect of outstanding options (determined using treasury stock method)									189	214	312
Dilutive effect of convertible debt (determined using treasury stock method)									0	0	546
Weighted-average common shares and potential common shares outstanding	104,356	107,048	111,757	114,106	114,061	114,158	114,188	114,124	109,240	114,077	113,058
Anti-dilutive stock options not included in treasury stock method computation									2,426	4,451	5,376