Summarized Quarterly Financial Data (Unaudited) (Details) (USD $) In Thousands, except Per Share data, 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
Summarized Quarterly Financial Data (Unaudited) [Abstract]
Operating revenues	$ 2,568,410	$ 2,831,598	$ 2,438,222	$ 2,816,540	$ 2,566,934	$ 2,628,787	$ 2,307,764	$ 2,807,462	$ 10,654,770	$ 10,310,947	$ 9,644,303
Operating income	344,001	651,496	359,442	426,663	322,370	568,630	325,304	403,665	1,781,602	1,619,969	1,468,572
Income from continuing operations	140,941	338,295	158,671	203,467	136,503	312,488	135,625	167,340	841,374	751,956	685,524
Discontinued operations - income (loss)	(432)	37	91	102	131	(182)	4,151	(222)	(202)	3,878	(4,637)
Net income	140,509	338,332	158,762	203,569	136,634	312,306	139,776	167,118	841,172	755,834	680,887
Earnings available to common shareholders	$ 140,509	$ 333,658	$ 157,702	$ 202,509	$ 135,573	$ 311,246	$ 138,716	$ 166,058	$ 834,378	$ 751,593	$ 676,646
Earnings per share total - basic (in dollars per share)	$ 0.29	$ 0.69	$ 0.33	$ 0.42	$ 0.29	$ 0.68	$ 0.30	$ 0.36	$ 1.72	$ 1.63	$ 1.48
Earnings per share total - diluted (in dollars per share)	$ 0.29	$ 0.69	$ 0.33	$ 0.42	$ 0.29	$ 0.67	$ 0.30	$ 0.36	$ 1.72	$ 1.62	$ 1.48
Cash dividends declared per common share (in dollars per share)	$ 0.26	$ 0.26	$ 0.26	$ 0.25	$ 0.25	$ 0.25	$ 0.25	$ 0.25	$ 1.03	$ 1.00	$ 0.97