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
Numerator:
Net income	$ 539	$ 503	$ 462	$ 594	$ 177	$ 507	$ 393	$ 403	$ 2,098	$ 1,480	$ 1,313
Denominator:
Basic: Weighted-average common shares outstanding	141.5	144.4	147.0	149.5	151.0	153.0	155.1	157.5	145.6	154.1	166.3
Effect of potentially dilutive nonqualified stock options and other share-based awards (in shares)									2.1	2.6	1.9
Diluted: Weighted-average common shares outstanding	143.2	146.5	149.0	152.1	153.8	155.4	157.5	160.1	147.7	156.7	168.2
Earnings Per Share, Basic:
Net income (in dollars per basic share)	$ 3.81	$ 3.48	$ 3.14	$ 3.97	$ 1.17	$ 3.31	$ 2.53	$ 2.56	$ 14.41	$ 9.60	$ 7.90
Earnings Per Share, Diluted:
Net income (in dollars per diluted share)	$ 3.76	$ 3.43	$ 3.10	$ 3.91	$ 1.15	$ 3.26	$ 2.50	$ 2.52	$ 14.20	$ 9.44	$ 7.81
Effect of potentially dilutive nonqualified stock options and other share-based awards (in shares)									3.2	0.0	1.5