Earnings per Share (details) - USD ($) $ / shares in Units, shares in Millions, $ in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2017	Sep. 30, 2016	Sep. 30, 2017	Sep. 30, 2016
Earnings per Share disclosure
Net income	$ 293	$ 716	$ 1,505	$ 2,071
Participating share-based awards - allocated income	(2)	(6)	(11)	(16)
Net income available to common shareholders -- basic	291	710	1,494	2,055
Net income available to common shareholders -- diluted	$ 291	$ 710	$ 1,494	$ 2,055
Weighted average shares outstanding, basic (in shares)	274.1	286.0	277.1	290.0
Weighted average effects of dilutive securities, stock options and performance shares (in shares)	2.5	3.8	2.5	3.6
Weighted average shares outstanding, diluted (in shares)	276.6	289.8	279.6	293.6
Net income per common share, basic (in dollars per share)	$ 1.06	$ 2.48	$ 5.39	$ 7.09
Net income per common share, diluted (in dollars per share)	$ 1.05	$ 2.45	$ 5.34	$ 7.00