Earnings per Share (details) - USD ($) $ / shares in Units, shares in Millions, $ in Millions	3 Months Ended		6 Months Ended
	Jun. 30, 2016	Jun. 30, 2015	Jun. 30, 2016	Jun. 30, 2015
Earnings per Share disclosure
Net income	$ 664	$ 812	$ 1,355	$ 1,645
Participating share-based awards - allocated income	(5)	(6)	(10)	(12)
Net income available to common shareholders -- basic	659	806	1,345	1,633
Net income available to common shareholders -- diluted	$ 659	$ 806	$ 1,345	$ 1,633
Weighted average shares outstanding, basic	290.1	314.8	292.1	317.7
Weighted average effects of dilutive securities, stock options and performance shares (in shares)	3.5	3.2	3.5	3.5
Weighted average shares outstanding, diluted	293.6	318.0	295.6	321.2
Net income per common share, basic	$ 2.27	$ 2.56	$ 4.60	$ 5.14
Net income per common share, diluted	$ 2.24	$ 2.53	$ 4.55	$ 5.08