Quarterly Financial Data (Unaudited) (Details) (USD $) In Millions, except Per Share data, unless otherwise specified	3 Months Ended								6 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	Jun. 30, 2012	Dec. 31, 2012	Dec. 31, 2011	Dec. 31, 2010
Quarterly Financial Data (Unaudited)
Operating revenues	$ 4,583	$ 4,571	$ 4,612	$ 4,610	$ 4,653	$ 4,596	$ 4,406	$ 1,696		$ 18,376	$ 15,351	$ 7,042
Operating income	666	736	657	654	533	548	480	464		2,713	2,025	2,060
Net income	233	270	74	200	109	138	115	211		777	573	948
Basic earnings per common share (in dollars per share)	$ 0.37	$ 0.43	$ 0.12	$ 0.32	$ 0.18	$ 0.22	$ 0.19	$ 0.69		$ 1.25	$ 1.07	$ 3.13
Diluted earnings per common share (in dollars per share)	$ 0.37	$ 0.43	$ 0.12	$ 0.32	$ 0.18	$ 0.22	$ 0.19	$ 0.69		$ 1.25	$ 1.07	$ 3.13
Quarterly Financial Data (Unaudited)
Depreciation expense									15	3,098	2,601	1,228
Correction of Immaterial Error
Quarterly Financial Data (Unaudited)
Depreciation expense		$ (45)							$ 15		$ 30