Share-Based Compensation Plans (Summary of Performance Share Activity) (Details) - Performance Shares - USD ($) $ / shares in Units, $ in Millions	12 Months Ended
	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
Performance Share Units
Outstanding, beginning of period (in shares)	1,902,502
Granted (in shares)	287,914
Forfeited (in shares)	0
Issued (in shares)	(461,942)
Cash settled (in shares)	(287,914)
Outstanding, end of period (in shares)	1,440,560	1,902,502
Not Vested, end of period (in shares)	73,271
Weighted-Average Grant-Date Fair Value
Outstanding, beginning of period (in dollars per share)	$ 49.52
Granted (in dollars per share)	87.42	$ 99.74	$ 86.88
Forfeited (in dollars per share)	0
Issued (in dollars per share)	59.12
Cash settled (in dollars per share)	87.42
Outstanding, end of period (in dollars per share)	46.44	$ 49.52
Not Vested, end of period (in dollars per share)	$ 69.63
Total Fair Value, Issued	$ 44	$ 70	$ 54
Total Fair Value, Cash settled	$ 25	$ 49	$ 56