Earnings per Share (details) - USD ($) $ / shares in Units, shares in Millions, $ in Millions	3 Months Ended								12 Months Ended
	Dec. 31, 2015	Sep. 30, 2015	Jun. 30, 2015	Mar. 31, 2015	Dec. 31, 2014	Sep. 30, 2014	Jun. 30, 2014	Mar. 31, 2014	Dec. 31, 2015	Dec. 31, 2014	Dec. 31, 2013
Earnings per Share disclosure
Net income	$ 866	$ 928	$ 812	$ 833	$ 1,038	$ 919	$ 683	$ 1,052	$ 3,439	$ 3,692	$ 3,673
Participating share-based awards - allocated income									(25)	(27)	(27)
Net income available to common shareholders -- basic									3,414	3,665	3,646
Net income available to common shareholders -- diluted									$ 3,414	$ 3,665	$ 3,646
Weighted average shares outstanding, basic									310.6	338.8	370.3
Weighted average effects of dilutive securities, stock options and performance shares (in shares)									3.3	3.7	4.0
Weighted average shares outstanding, diluted									313.9	342.5	374.3
Net income per common share, basic	$ 2.87	$ 3.00	$ 2.56	$ 2.58	$ 3.15	$ 2.72	$ 1.98	$ 2.98	$ 10.99	$ 10.82	$ 9.84
Net income per common share, diluted	$ 2.83	$ 2.97	$ 2.53	$ 2.55	$ 3.11	$ 2.69	$ 1.95	$ 2.95	$ 10.88	$ 10.70	$ 9.74