Earnings per Share (Details) - USD ($) $ / shares in Units, shares in Millions, $ in Millions	3 Months Ended								12 Months Ended
	Dec. 31, 2019	Sep. 30, 2019	Jun. 30, 2019	Mar. 31, 2019	Dec. 31, 2018	Sep. 30, 2018	Jun. 30, 2018	Mar. 31, 2018	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
Numerator:
Net income	$ 463	$ 543	$ 492	$ 395	$ 539	$ 503	$ 462	$ 594	$ 1,893	$ 2,098	$ 1,480
Denominator:
Basic: Weighted-average common shares outstanding	129.0	132.7	136.1	138.8	141.5	144.4	147.0	149.5	134.1	145.6	154.1
Effect of potentially dilutive nonqualified stock options and other share-based awards (in shares)									1.9	2.1	2.6
Diluted: Weighted-average common shares outstanding	131.3	134.5	138.0	140.1	143.2	146.5	149.0	152.1	136.0	147.7	156.7
Earnings Per Share, Basic:
Net income (in dollars per basic share)	$ 3.59	$ 4.09	$ 3.61	$ 2.85	$ 3.81	$ 3.48	$ 3.14	$ 3.97	$ 14.12	$ 14.41	$ 9.60
Earnings Per Share, Diluted:
Net income (in dollars per diluted share)	$ 3.53	$ 4.04	$ 3.57	$ 2.82	$ 3.76	$ 3.43	$ 3.10	$ 3.91	$ 13.92	$ 14.20	$ 9.44
Effect of potentially dilutive nonqualified stock options and other share-based awards (in shares)									1.0	3.2	0.0