Earnings per Share (Details) - USD ($) $ / shares in Units, shares in Millions, $ in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2018	Sep. 30, 2017	Sep. 30, 2018	Sep. 30, 2017
Earnings Per Share Reconciliation [Abstract]
Net income	$ 709	$ 293	$ 1,902	$ 1,505
Participating share-based awards — allocated income	(5)	(2)	(14)	(11)
Net income available to common shareholders -- basic	704	291	1,888	1,494
Net income available to common shareholders -- diluted	$ 704	$ 291	$ 1,888	$ 1,494
Weighted average shares outstanding, basic (in shares)	266.1	274.1	268.6	277.1
Weighted average effects of dilutive securities - stock options and performance shares (in shares)	2.3	2.5	2.5	2.5
Weighted average shares outstanding, diluted (in shares)	268.4	276.6	271.1	279.6
Net income per common share, basic (in dollars per share)	$ 2.65	$ 1.06	$ 7.03	$ 5.39
Net income per common share, diluted (in dollars per share)	$ 2.62	$ 1.05	$ 6.97	$ 5.34