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Incentive Plan Incentive Plan (Schedule of Deferred and Unearned Compensation) (Details) - USD ($)
$ in Millions
3 Months Ended
Mar. 31, 2018
Mar. 31, 2017
Dec. 31, 2017
Long-term incentive units      
Deferred Compensation Arrangement with Individual, Share-based Payments [Line Items]      
Deferred compensation, expensed $ 0.9 $ 0.7  
Deferred compensation, capitalized 0.2 0.4  
Unearned compensation $ 15.0   $ 6.9
Expected period to recognize unearned compensation (in years) 3 years 2 months 10 days    
Market performance-based awards      
Deferred Compensation Arrangement with Individual, Share-based Payments [Line Items]      
Deferred compensation, expensed $ 3.1 2.1  
Deferred compensation, capitalized 0.3 0.7  
Unearned compensation $ 44.7   24.7
Expected period to recognize unearned compensation (in years) 2 years 6 months 19 days    
Restricted stock      
Deferred Compensation Arrangement with Individual, Share-based Payments [Line Items]      
Deferred compensation, expensed $ 1.5 1.0  
Deferred compensation, capitalized 1.1 $ 0.8  
Unearned compensation $ 30.5   $ 17.5
Expected period to recognize unearned compensation (in years) 3 years 2 months 15 days