Share-Based Compensation Plans - Stock Option Activity (Details) - USD ($) $ / shares in Units, shares in Thousands, $ in Millions	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Shares
Outstanding at beginning of period (in shares)	574	1,127	1,272
Granted (in shares)	122	145	166
Exercised (in shares)	(24)	(680)	(279)
Expired or cancelled (in shares)	0	(18)	(32)
Outstanding at end of period (in shares)	672	574	1,127
Vested or expected to vest (in shares)	670	571	1,122
Exercisable (in shares)	401	220	756
Wtd. Avg. Exercise Price Per Share
Outstanding at beginning of period (in dollars per share)	$ 77	$ 66	$ 64
Granted (in dollars per share)	123	97	75
Exercised (in dollars per share)	78	64	63
Expired or cancelled (in dollars per share)	0	75	67
Outstanding at end of period (in dollars per share)	85	77	66
Vested or expected to vest (in dollars per share)	85	77	66
Exercisable (in dollars per share)	$ 76	$ 65	$ 62
Wtd. Avg. Remaining Contractual Term and Aggregate Intrinsic Value
Outstanding (in years)	5 years 10 months 24 days	7 years 1 month 6 days	4 years 10 months 24 days
Vested or expected to vest (in years)	5 years 10 months 24 days	7 years 1 month 6 days	4 years 10 months 24 days
Exercisable (in years)	4 years 6 months	5 years 3 months 18 days	3 years 3 months 18 days
Exercised	$ 1	$ 24	$ 5
Outstanding	1	28	26
Exercisable	$ 1	$ 14	$ 20