Stock Compensation Plans - Nonvested Share Activity (Details) - USD ($) $ / shares in Units, $ in Millions	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Share-based Compensation
Stock-based compensation expense recognized	$ 22	$ 23	$ 19
Tax benefit	(6)	(8)	(7)
After-tax stock-based compensation expense	16	15	12
Cash payments to settle SARs	1	1	2
Cash received from stock option exercises	$ 19	$ 17	$ 53
Performance Share Units
Summary of Activity Related to Share-based Compensation
Unvested units, beginning of period (in shares)	369,303
Units granted (in shares)	164,996
Units vested (in shares)	(184,961)
Units forfeited (in shares)	(7,031)
Unvested units, end of period (in shares)	342,307	369,303
Weighted Average Grant Date Fair Value Related to Share-based Compensation
Unvested units, beginning of period (in dollars per share)	$ 29.68
Units granted (in dollars per share)	36.78
Units vested (in dollars per share)	28.44
Units forfeited (in dollars per share)	31.72
Unvested units, end of period (in dollars per share)	$ 33.73	$ 29.68
Restricted Stock Units
Summary of Activity Related to Share-based Compensation
Unvested units, beginning of period (in shares)	826,459
Units granted (in shares)	323,277
Units vested (in shares)	(196,361)
Units forfeited (in shares)	(96,977)
Unvested units, end of period (in shares)	856,398	826,459
Weighted Average Grant Date Fair Value Related to Share-based Compensation
Unvested units, beginning of period (in dollars per share)	$ 28.14
Units granted (in dollars per share)	33.58
Units vested (in dollars per share)	26.94
Units forfeited (in dollars per share)	30.93
Unvested units, end of period (in dollars per share)	$ 30.15	$ 28.14