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Stock-Based Compensation Programs - Weighted average assumptions (Details)
12 Months Ended
Dec. 31, 2017
Dec. 31, 2016
Weighted average assumptions used in estimation of fair values of options    
Expected dividend yield (as a percent) 0.89%  
Expected stock price volatility (as a percent) 19.62%  
Risk-free interest rate (as a percent) 2.00%  
Expected life of options 5 years 11 months 9 days  
Stock option and SSARs    
Weighted average assumptions used in estimation of fair values of options    
Expected dividend yield (as a percent) 0.73% 0.79%
Expected stock price volatility (as a percent) 24.14% 22.11%
Risk-free interest rate (as a percent) 1.22% 1.39%
Expected life of options 6 years 1 month 6 days 5 years 10 months 6 days