Earnings per Share (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
Numerator:
Net income	$ 177	$ (140)		$ (539)		$ 2,036	$ 463	$ 543	$ 492	$ 395	$ 1,534	$ 1,893	$ 2,098
Denominator:
Basic: Weighted-average common shares outstanding	120.9	123.0		125.0		126.4	129.0	132.7	136.1	138.8	123.8	134.1	145.6
Effect of potentially dilutive nonqualified stock options and other share-based awards (in shares)											1.9	1.9	2.1
Diluted: Weighted-average common shares outstanding	123.4	124.9		126.2		128.2	131.3	134.5	138.0	140.1	125.7	136.0	147.7
Earnings Per Share, Basic:
Net income (in dollars per basic share)	$ 1.46	$ (1.14)		$ (4.31)		$ 16.11	$ 3.59	$ 4.09	$ 3.61	$ 2.85	$ 12.39	$ 14.12	$ 14.41
Earnings Per Share, Diluted:
Net income (in dollars per diluted share)	$ 1.43	$ (1.14)	[1]	$ (4.31)	[1]	$ 15.88	$ 3.53	$ 4.04	$ 3.57	$ 2.82	$ 12.20	$ 13.92	$ 14.20
Effect of potentially dilutive nonqualified stock options and other share-based awards (in shares)											0.0	1.0	3.2

[1] Diluted shares used in this calculation represent basic shares due to the net loss. Using actual diluted shares would result in anti-dilution.