Quarterly Financial Data (Unaudited) (Details) (USD $) In Millions, 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
Quarterly Financial Information Disclosure [Abstract]
Operating revenue	$ 3,013		$ 3,981		$ 2,983		$ 2,782		$ 3,069	$ 3,788	$ 2,741	$ 2,810	$ 12,760		$ 12,409	$ 12,361
Operating income (loss)	(1,319)		810		380		450		414	862	351	498	320		2,126	1,398
Income (loss) from continuing operations	(824)		441		192		216		178	527	356	243	24		1,303	952
Income (loss) from discontinued operations, net of tax	0		0		(1)		(2)		0	(4)	1	6	(3)		4	(7)
Net income (loss) attributable to common shareholders	$ (839)		$ 426		$ 176		$ 200		$ 166	$ 510	$ 344	$ 236	$ (37)		$ 1,256	$ 849
Basic earnings (loss) per share:
Continuing operations (in dollars per share)	$ (2.57)		$ 1.31		$ 0.54		$ 0.62		$ 0.51	$ 1.57	$ 1.05	$ 0.70	$ (0.10)		$ 3.83	$ 2.61
Discontinued operations (in dollars per share)	$ 0.00		$ 0.00		$ 0.00		$ (0.01)		$ 0.00	$ (0.01)	$ 0.00	$ 0.02	$ (0.01)		$ 0.01	$ (0.02)
Total (in dollars per share)	$ (2.57)		$ 1.31		$ 0.54		$ 0.61		$ 0.51	$ 1.56	$ 1.05	$ 0.72	$ (0.11)		$ 3.84	$ 2.59
Diluted earnings (loss) per share:
Continuing operations (in dollars per share)	$ (2.57)	[1]	$ 1.30	[1]	$ 0.54	[1]	$ 0.62	[1]	$ 0.51	$ 1.57	$ 1.05	$ 0.70	$ (0.10)	[1]	$ 3.81	$ 2.60
Discontinued operations (in dollars per share)	$ 0.00	[1]	$ 0.00	[1]	$ 0.00	[1]	$ (0.01)	[1]	$ 0.00	$ (0.01)	$ 0.00	$ 0.02	$ (0.01)	[1]	$ 0.01	$ (0.02)
Total (in dollars per share)	$ (2.57)	[1]	$ 1.30	[1]	$ 0.54	[1]	$ 0.61	[1]	$ 0.51	$ 1.56	$ 1.05	$ 0.72	$ (0.11)	[1]	$ 3.82	$ 2.58
Dividends declared per common share (in dollars per share)	$ 0.325	[1]	$ 0.320	[1]	$ 0.320	[1]	$ 0.320	[1]	$ 0.320	$ 0.315	$ 0.315	$ 0.315	$ 1.285	[1]	$ 1.265	$ 1.245
Common stock prices:
High (in dollars per share)	$ 41.57		$ 39.25		$ 40.15		$ 39.20		$ 39.37	$ 35.15	$ 34.74	$ 35.82	$ 41.57		$ 39.37
Low (in dollars per share)	$ 35.63		$ 32.64		$ 36.54		$ 35.12		$ 34.38	$ 31.06	$ 30.37	$ 31.88	$ 32.64		$ 30.37
Close (in dollars per share)	$ 41.40		$ 38.25		$ 38.75		$ 36.59		$ 38.60	$ 34.39	$ 31.72	$ 34.17	$ 41.40		$ 38.60

[1] Due to a loss for the three months and year ended December 31, 2011, there are no incremental shares in the diluted computation because such shares would be considered antidilutive.