Earnings Per Share (Details) (USD $) In Thousands, except Per Share data, unless otherwise specified	3 Months Ended		9 Months Ended
	Sep. 30, 2012	Sep. 30, 2011	Sep. 30, 2012	Sep. 30, 2011
Earnings Per Share
Net income for basic and diluted earnings per share (in dollars)	$ 367,779	$ 398,958	$ 411,090	$ 570,859
Weighted-average common shares outstanding	217,940	217,105	217,797	216,883
Dilutive effect of stock options and restricted stock awards (in shares)	1,356	1,829	1,466	2,432
Diluted weighted-average shares outstanding	219,296	218,934	219,263	219,315
Basic earnings per share (in dollars per share)	$ 1.69	$ 1.84	$ 1.89	$ 2.63
Diluted earnings per share (in dollars per share)	$ 1.68	$ 1.82	$ 1.87	$ 2.60