Stock Compensation Plans - Nonvested Share Activity (Details) - USD ($) $ / shares in Units, $ in Millions	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Share-based Compensation
Stock-based compensation expense recognized	$ 16	$ 22	$ 23
Tax benefit	(4)	(6)	(8)
After-tax stock-based compensation expense	12	16	15
Cash payments to settle SARs	1	1	1
Cash received from stock option exercises	$ 9	$ 19	$ 17
Performance Share Units
Summary of Activity Related to Share-based Compensation
Unvested units, beginning of period (in shares)	342,307
Units granted (in shares)	195,583
Units vested (in shares)	(157,396)
Units forfeited (in shares)	(47,298)
Unvested units, end of period (in shares)	333,196	342,307
Weighted Average Grant Date Fair Value Related to Share-based Compensation
Unvested units, beginning of period (in dollars per share)	$ 33.73
Units granted (in dollars per share)	29.04
Units vested (in dollars per share)	30.95
Units forfeited (in dollars per share)	33.63
Unvested units, end of period (in dollars per share)	$ 32.30	$ 33.73
Restricted Stock Units
Summary of Activity Related to Share-based Compensation
Unvested units, beginning of period (in shares)	856,398
Units granted (in shares)	935,656
Units vested (in shares)	(207,542)
Units forfeited (in shares)	(196,079)
Unvested units, end of period (in shares)	1,388,433	856,398
Weighted Average Grant Date Fair Value Related to Share-based Compensation
Unvested units, beginning of period (in dollars per share)	$ 30.15
Units granted (in dollars per share)	28.54
Units vested (in dollars per share)	28.06
Units forfeited (in dollars per share)	28.15
Unvested units, end of period (in dollars per share)	$ 29.81	$ 30.15