Share-Based Compensation Plans - Restricted Stock Activity (Details) - Restricted stock - USD ($) $ / shares in Units, shares in Thousands, $ in Millions	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Shares
Unvested at beginning of period (in shares)	448	451	405
Granted (in shares)	145	167	232
Vested (in shares)	(174)	(133)	(172)
Forfeited (in shares)	(23)	(37)	(14)
Unvested at end of period (in shares)	396	448	451	405
Wtd. Avg. Price Per Share
Unvested at beginning of period (in dollars per share)	$ 77	$ 70	$ 64
Granted (in dollars per share)	119	98	75
Vested (in dollars per share)	77	79	62
Forfeited (in dollars per share)	89	80	76
Unvested at end of period (in dollars per share)	$ 92	$ 77	$ 70	$ 64
Equity Other Than Options, Additional [Abstract]
Unvested (in years)	1 year 3 months 18 days	1 year 2 months 12 days	1 year 4 months 24 days	1 year 3 months 18 days
Unvested, aggregate intrinsic value	$ 26