Net Income 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
Earnings Per Share [Abstract]
Net Income	$ 641	$ 660	$ 623	$ 597	$ 607	$ 638	$ 666	$ 652	$ 2,521	$ 2,563	$ 1,687
Net income per share—Basic:
Weighted-average common shares (in shares)									323.9	335.0	344.1
Net income per share—Basic (in dollars per share)	$ 2.00	$ 2.05	$ 1.92	$ 1.82	$ 1.84	$ 1.91	$ 1.98	$ 1.92	$ 7.78	$ 7.65	$ 4.90
Net income per share—Diluted:
Weighted-average common shares (in shares)									323.9	335.0	344.1
Effect of dilutive stock options and restricted stock units (in shares)									1.7	2.1	2.7
Weighted-average common shares assuming dilution (in shares)									325.6	337.1	346.8
Net income per share—Diluted (in dollars per share)	$ 1.99	$ 2.04	$ 1.91	$ 1.81	$ 1.83	$ 1.90	$ 1.97	$ 1.90	$ 7.74	$ 7.60	$ 4.86