Earnings per Share (Details) - USD ($) $ / shares in Units, shares in Millions, $ in Millions	3 Months Ended		6 Months Ended
	Jun. 30, 2018	Jun. 30, 2017	Jun. 30, 2018	Jun. 30, 2017
Earnings Per Share Reconciliation [Abstract]
Net income	$ 524	$ 595	$ 1,193	$ 1,212
Participating share-based awards — allocated income	(4)	(5)	(9)	(9)
Net income available to common shareholders -- basic	520	590	1,184	1,203
Net income available to common shareholders -- diluted	$ 520	$ 590	$ 1,184	$ 1,203
Weighted average shares outstanding, basic (in shares)	268.7	277.5	269.8	278.6
Weighted average effects of dilutive securities - stock options and performance shares (in shares)	2.4	2.5	2.7	2.6
Weighted average shares outstanding, diluted (in shares)	271.1	280.0	272.5	281.2
Net income per common share, basic (in dollars per share)	$ 1.93	$ 2.13	$ 4.39	$ 4.32
Net income per common share, diluted (in dollars per share)	$ 1.92	$ 2.11	$ 4.35	$ 4.28