Earnings per Share (Details) - USD ($) $ / shares in Units, shares in Millions, $ in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2019	Sep. 30, 2018	Sep. 30, 2019	Sep. 30, 2018
Earnings Per Share Reconciliation [Abstract]
Net income	$ 396	$ 709	$ 1,749	$ 1,902
Participating share-based awards — allocated income	(3)	(5)	(12)	(14)
Net income available to common shareholders -- basic	393	704	1,737	1,888
Net income available to common shareholders -- diluted	$ 393	$ 704	$ 1,737	$ 1,888
Weighted average shares outstanding, basic (in shares)	259.2	266.1	261.1	268.6
Weighted average effects of dilutive securities - stock options and performance shares (in shares)	2.6	2.3	2.3	2.5
Weighted average shares outstanding, diluted (in shares)	261.8	268.4	263.4	271.1
Net income per common share, basic (in dollars per share)	$ 1.52	$ 2.65	$ 6.65	$ 7.03
Net income per common share, diluted (in dollars per share)	$ 1.50	$ 2.62	$ 6.59	$ 6.97