Quarterly Financial Data (Unaudited) - Quarterly Financial Information (Details) (USD $) In Millions, except Per Share data, unless otherwise specified	3 Months Ended								12 Months Ended
	Dec. 31, 2014	Sep. 30, 2014	Jun. 30, 2014	Mar. 31, 2014	Dec. 31, 2013	Sep. 30, 2013	Jun. 30, 2013	Mar. 31, 2013	Dec. 31, 2014	Dec. 31, 2013	Dec. 31, 2012
Quarterly Financial Information Disclosure [Abstract]
Operating revenue	$ 9,647	$ 11,178	$ 10,621	$ 8,916	$ 9,076	$ 10,490	$ 9,707	$ 8,500	$ 40,362	$ 37,773	$ 36,670
Operating income (loss)	(828)	835	1,579	620	701	1,563	914	222	2,206	3,400	2,175
Net Income	(712)	357	801	213	8,479	1,369	685	7	659	10,540	1,009
Basic earnings (loss) per share	$ (0.86)	$ 0.43	$ 0.95	$ 0.25	$ 10.02	$ 1.61	$ 0.81	$ 0.01	$ 0.79	$ 12.41	$ 1.20
Diluted earnings (loss) per share	$ (0.86)	$ 0.42	$ 0.94	$ 0.25	$ 9.89	$ 1.59	$ 0.80	$ 0.01	$ 0.78	$ 12.29	$ 1.19
Quarterly financial information, quarterly charges and credits, amount affecting comparability	$ (2,157)	$ (1,058)	$ (140)	$ (109)	$ 7,921	$ 157	$ (159)	$ (78)