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Stock Compensation Plans - Performance Shares (Details) - Performance Shares
12 Months Ended
Dec. 31, 2017
Dec. 31, 2016
Dec. 31, 2015
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]      
Volatility of common stock 20.30% 22.20% 21.40%
Average correlation coefficient of peer companies 60.00% 56.00% 54.00%
Risk-free spot rate 1.50% 1.00% 1.10%
Term 3 years 3 years 3 years
Minimum      
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]      
Average volatility of peer companies 15.00% 15.00% 14.00%
Maximum      
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]      
Average volatility of peer companies 25.00% 26.00% 24.00%