Earnings Per Share - Computation of 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 [Abstract]
Earnings available to common stockholders	$ 836	$ 645		$ 1,667	$ 1,565
Basic weighted-average shares outstanding (in shares)	272.4	299.8		277.0	306.8
Effect of employee stock awards (in shares)	1.2	0.8		1.0	0.8
Diluted weighted-average shares outstanding (in shares)	273.6	300.6		278.0	307.6
Earnings per share, basic (in dollars per share)	$ 3.07	$ 2.15	[1]	$ 6.02	$ 5.10	[1]
Earnings per share, diluted (in dollars per share)	$ 3.06	$ 2.15	[1]	$ 5.99	$ 5.09	[1]

[1] Amounts adjusted due to the adoption of Accounting Standards Update No. 2014-09, Revenue from Contracts with Customers (Topic 606) and Accounting Standards Update No. 2017-07, Improving the Presentation of Net Periodic Pension Cost and Net Periodic Postretirement Benefit Cost. See Note 1 to the financial statements contained in Part I, Item 1 of this report for additional information.