Earnings per Share (Details) - USD ($) $ / shares in Units, shares in Millions, $ in Millions	3 Months Ended
	Mar. 31, 2018	Mar. 31, 2017
Earnings Per Share Reconciliation [Abstract]
Net income	$ 669	$ 617
Participating share-based awards — allocated income	(5)	(4)
Net income available to common shareholders -- basic	664	613
Net income available to common shareholders -- diluted	$ 664	$ 613
Weighted average shares outstanding, basic (in shares)	271.0	279.7
Weighted average effects of dilutive securities - stock options and performance shares (in shares)	2.9	2.7
Weighted average shares outstanding, diluted (in shares)	273.9	282.4
Net income per common share, basic (in dollars per share)	$ 2.45	$ 2.19
Net income per common share, diluted (in dollars per share)	$ 2.42	$ 2.17