Share-Based Compensation Plans - RTSR fair value assumptions (Details)	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Dividend yield	0.80%	0.90%	0.50%
Volatility, as a percent	31.20%	30.00%	27.60%
Risk-free interest rate	2.80%	1.80%	1.30%
Expected life in years	5 years 7 months 6 days	5 years 4 months 24 days	5 years 6 months
RTSR
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Dividend yield	0.90%	0.90%
Volatility, as a percent	35.30%	36.10%
Risk-free interest rate	2.40%	1.40%
Expected life in years	2 years 9 months 18 days	2 years 7 months 6 days