Earnings per Share (details) - USD ($) $ / shares in Units, shares in Millions, $ in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2016	Sep. 30, 2015	Sep. 30, 2016	Sep. 30, 2015
Earnings per Share disclosure
Net income	$ 716	$ 928	$ 2,071	$ 2,573
Participating share-based awards - allocated income	(6)	(6)	(16)	(18)
Net income available to common shareholders -- basic	710	922	2,055	2,555
Net income available to common shareholders -- diluted	$ 710	$ 922	$ 2,055	$ 2,555
Weighted average shares outstanding, basic	286.0	307.6	290.0	314.3
Weighted average effects of dilutive securities, stock options and performance shares (in shares)	3.8	3.4	3.6	3.4
Weighted average shares outstanding, diluted	289.8	311.0	293.6	317.7
Net income per common share, basic	$ 2.48	$ 3.00	$ 7.09	$ 8.13
Net income per common share, diluted	$ 2.45	$ 2.97	$ 7.00	$ 8.04