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Stock Incentive Plans - Fair Value Valuation Assumptions (Details) - Performance Stock Units - $ / shares
12 Months Ended
Dec. 31, 2017
Dec. 31, 2016
Dec. 31, 2015
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]      
Fair Value at Grant Date (in dollars per share) $ 11.04 $ 8.54 $ 12.13
February 2015 Award      
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]      
Volatility 22.90%    
Risk-Free Rate 1.01%    
Fair Value at Grant Date (in dollars per share) $ 12.13    
February 2016 Award      
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]      
Volatility 24.30%    
Risk-Free Rate 0.93%    
Fair Value at Grant Date (in dollars per share) $ 8.42    
February 2017 Award      
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]      
Volatility 26.70%    
Risk-Free Rate 1.46%    
Fair Value at Grant Date (in dollars per share) $ 10.89