Earnings Per Common Share ("EPS") (Details) - USD ($) $ / shares in Units, shares in Millions, $ in Millions		3 Months Ended								12 Months Ended
		Dec. 31, 2020	Sep. 30, 2020	Jun. 30, 2020	Mar. 31, 2020	Dec. 31, 2019	Sep. 30, 2019	Jun. 30, 2019	Mar. 31, 2019	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
Earnings Per Share [Abstract]
Net Income		$ 332	$ 283	$ 206	$ 83	$ 488	$ 255	$ 289	$ 262	$ 904	$ 1,294	$ 1,542
Weighted-average common shares outstanding (for basic calculation) (in shares)										302.0	306.0	322.0
Effect of dilutive share-based employee compensation (in shares)										5.0	7.0	7.0
Weighted-average common and dilutive potential common shares outstanding (for diluted calculation) (in shares)										307.0	313.0	329.0
Basic EPS (in dollars per share)		$ 1.10	$ 0.94	$ 0.68	$ 0.28	$ 1.61	$ 0.83	$ 0.94	$ 0.85	$ 2.99	$ 4.23	$ 4.80
Diluted EPS (in dollars per share)		$ 1.08	$ 0.92	$ 0.67	$ 0.27	$ 1.58	$ 0.81	$ 0.92	$ 0.83	$ 2.94	$ 4.14	$ 4.69
Unexercised employee stock options and stock appreciation rights (in millions) excluded from the diluted EPS computation (in shares)	[1]									4.8	2.0	2.0

[1] These unexercised employee stock options and stock appreciation rights were not included in the computation of diluted EPS because to do so would have been antidilutive for the periods presented.