Earnings per Share (Details) - USD ($) $ / shares in Units, shares in Millions, $ in Millions	3 Months Ended								12 Months Ended
	Dec. 31, 2018	Sep. 30, 2018	Jun. 30, 2018	Mar. 31, 2018	Dec. 31, 2017	Sep. 30, 2017	Jun. 30, 2017	Mar. 31, 2017	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Earnings Per Share Reconciliation [Abstract]
Net income	$ 621	$ 709	$ 524	$ 669	$ 551	$ 293	$ 595	$ 617	$ 2,523	$ 2,056	$ 3,014
Participating share-based awards — allocated income									(19)	(15)	(22)
Net income available to common shareholders — basic									2,504	2,041	2,992
Net income available to common shareholders — diluted									$ 2,504	$ 2,041	$ 2,992
Weighted average shares outstanding, basic (in shares)									267.4	276.0	288.1
Weighted average effects of dilutive securities, stock options and performance shares (in shares)									2.4	2.6	2.9
Weighted average shares outstanding, diluted (in shares)									269.8	278.6	291.0
Net income per common share, basic (in dollars per share)	$ 2.33	$ 2.65	$ 1.93	$ 2.45	$ 2.00	$ 1.06	$ 2.13	$ 2.19	$ 9.37	$ 7.39	$ 10.39
Net income per common share, diluted (in dollars per share)	$ 2.32	$ 2.62	$ 1.92	$ 2.42	$ 1.98	$ 1.05	$ 2.11	$ 2.17	$ 9.28	$ 7.33	$ 10.28