Share-Based Compensation Plans - RTSR fair value assumptions (Details)	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Jun. 27, 2015
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Dividend yield	0.90%	0.50%	0.30%
Volatility, as a percent	30.00%	27.60%	27.10%
Risk-free interest rate	1.80%	1.30%	1.70%
Expected life in years	5 years 4 months 28 days	5 years 5 months 27 days	5 years 3 months 18 days
RTSR
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Dividend yield	0.90%
Volatility, as a percent	36.10%
Risk-free interest rate	1.40%
Expected life in years	2 years 6 months 26 days