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Compensation Plans (Details)
$ in Millions
3 Months Ended
Mar. 31, 2018
USD ($)
shares
Mar. 31, 2017
USD ($)
Performance Shares [Member]    
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]    
Share-based Compensation Arrangement by Share-based Payment Award, Award Vesting Period 3 years  
Stock-based compensation expense $ 2.4 $ 2.5
Unrecognized stock based compensation expense $ 15.6  
Performance Shares [Member] | Minimum [Member]    
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]    
Multiplier applied to PSU awards at settlement 0  
Performance Shares [Member] | Maximum [Member]    
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]    
Multiplier applied to PSU awards at settlement 2  
Restricted Stock Units (RSUs) [Member]    
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]    
Stock-based compensation expense $ 2.7 $ 2.5
Unrecognized stock based compensation expense $ 16.2  
Number of shares represented by each RSU | shares 1