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Stock-Based Compensation (Details) - USD ($)
$ / shares in Units, $ in Millions
3 Months Ended
Mar. 31, 2018
Mar. 31, 2017
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]    
Share-based Compensation Arrangement by Share-based Payment Award, Options, Grants in Period, Gross 24,932  
Stock-Based Compensation Activity    
Income tax benefit $ 0.7 $ 0.8
Performance Shares [Member]    
Stock-Based Compensation Activity    
Compensation expense $ 2.7 2.1
Total Shareholder Return [Member]    
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]    
Share-based Compensation Arrangement by Share-based Payment Award, Equity Instruments Other than Options, Grants in Period, Weighted Average Grant Date Fair Value $ 36.86  
Stock-Based Compensation Activity    
Share-based Compensation Arrangement by Share-based Payment Award, Non-Option Equity Instruments, Granted 261,916  
Performance Units Related to Earnings Per Share [Member]    
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]    
Share-based Compensation Arrangement by Share-based Payment Award, Equity Instruments Other than Options, Grants in Period, Weighted Average Grant Date Fair Value $ 31.03  
Stock-Based Compensation Activity    
Share-based Compensation Arrangement by Share-based Payment Award, Non-Option Equity Instruments, Granted 87,308  
Restricted Stock [Member]    
Stock-Based Compensation Activity    
Compensation expense $ 0.0 0.0
Stock Compensation Plan [Member]    
Stock-Based Compensation Activity    
Compensation expense 2.7 2.1
Total Shareholder Return [Member] | Performance Shares [Member]    
Stock-Based Compensation Activity    
Compensation expense 2.0 1.5
Performance Units Related to Earnings Per Share [Member] | Performance Shares [Member]    
Stock-Based Compensation Activity    
Compensation expense $ 0.7 $ 0.6