Net Income 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
Earnings Per Share [Abstract]
Net Income	$ 607	$ 638	$ 666	$ 652	$ (76)	$ 640	$ 587	$ 536	$ 2,563	$ 1,687	$ 2,035
Net income per share—Basic:
Weighted-average common shares (in shares)									335.0	344.1	355.0
Net income per share—Basic (in dollars per share)	$ 1.84	$ 1.91	$ 1.98	$ 1.92	$ (0.22)	$ 1.86	$ 1.70	$ 1.55	$ 7.65	$ 4.90	$ 5.73
Net income per share—Diluted:
Weighted-average common shares (in shares)									335.0	344.1	355.0
Effect of dilutive stock options and restricted stock units (in shares)									2.1	2.7	2.1
Weighted-average common shares assuming dilution (in shares)									337.1	346.8	357.1
Net income per share—Diluted (in dollars per share)	$ 1.83	$ 1.90	$ 1.97	$ 1.90	$ (0.22)	$ 1.85	$ 1.69	$ 1.54	$ 7.60	$ 4.86	$ 5.70