Earnings per Share (details) (USD $) In Millions, except Per Share data, unless otherwise specified	3 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	Dec. 31, 2012	Dec. 31, 2011	Dec. 31, 2010
Earnings per Share disclosure
Net income	$ 304	$ 864	$ 499	$ 806	$ 618	$ 333	$ (364)	$ 839	$ 2,473	$ 1,426	$ 3,216
Participating share-based awards - allocated income									(19)	(11)	(25)
Preferred stock dividends										(1)	(3)
Net income available to common shareholders -- basic									2,454	1,414	3,188
Effect of dilutive securities, participating share-based awards - re-allocated income											2
Effect of dilutive securities, convertible preferred stock										1	3
Net income available to common shareholders -- diluted									$ 2,454	$ 1,415	$ 3,193
Weighted average shares outstanding, basic									386.2	415.8	476.5
Weighted average effects of dilutive securities, stock options and performance shares (in shares)									3.6	4.0	4.2
Weighted average effects of dilutive securities, convertible preferred stock (in shares)										0.7	1.8
Weighted average shares outstanding, diluted									389.8	420.5	482.5
Net income per common share, basic	$ 0.79	$ 2.23	$ 1.27	$ 2.04	$ 1.52	$ 0.80	$ (0.88)	$ 1.94	$ 6.35	$ 3.40	$ 6.69
Net income per common share, diluted	$ 0.78	$ 2.21	$ 1.26	$ 2.02	$ 1.51	$ 0.79	$ (0.88)	$ 1.92	$ 6.30	$ 3.36	$ 6.62