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Retirement Plans and Post Retirement Benefits Level 3 Rollforward (Details) - USD ($)
$ in Millions
 12 Months Ended
Dec. 31, 2017
Dec. 31, 2016
Dec. 31, 2017
Assets Measured at Fair Value on a Recurring Basis Using Significant Unobservable Inputs (Level 3) [Roll Forward]      
Fair value of plan assets, beginning of year balance $ 239.9 $ 237.4  
Fair value of plan assets, end of year balance 239.9 237.4 $ 265.9
Multi-strategy Funds      
Assets Measured at Fair Value on a Recurring Basis Using Significant Unobservable Inputs (Level 3) [Roll Forward]      
Fair value of plan assets, beginning of year balance [1] 75.1    
Fair value of plan assets, end of year balance [1] 75.1   82.5
Significant Unobservable Inputs (Level 3)      
Assets Measured at Fair Value on a Recurring Basis Using Significant Unobservable Inputs (Level 3) [Roll Forward]      
Fair value of plan assets, beginning of year balance 12.2    
Fair value of plan assets, end of year balance 12.2   12.9
Significant Unobservable Inputs (Level 3) | Multi-strategy Funds      
Assets Measured at Fair Value on a Recurring Basis Using Significant Unobservable Inputs (Level 3) [Roll Forward]      
Fair value of plan assets, beginning of year balance 12.2 [1] 13.3  
Total unrealized gains (losses) 0.7 0.3  
Defined Benefit Plan, Redemptions 0.1 1.5  
Defined Benefit Plan, Mangement Fees (0.1) 0.1  
Fair value of plan assets, end of year balance $ 12.2 [1] $ 13.3 $ 12.9 [1]
[1] The multi-strategy funds invest in various hedge funds that employ a fund of funds strategy.