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Summary of Significant Accounting Policies Accumulated Other Comprehensive Income (Loss) (Details) - USD ($)
$ in Millions
3 Months Ended
Mar. 31, 2018
Mar. 31, 2017
Dec. 31, 2017
Dec. 31, 2016
Accumulated Other Comprehensive Income (Loss), Net of Tax $ (23.0) $ (28.7) $ (23.2) $ (29.3)
Reclassification from Accumulated Other Comprehensive Income, Current Period, Net of Tax 0.2 0.6    
Accumulated Defined Benefit Plans Adjustment, Net Unamortized Gain (Loss) [Member] | Pension Plan [Member]        
Accumulated Other Comprehensive (Income) Loss, Defined Benefit Plan, after Tax (32.0) (31.5) (32.7) (32.1)
Reclassification from Accumulated Other Comprehensive Income, Current Period, Net of Tax 0.7 0.6    
Accumulated Defined Benefit Plans Adjustment, Net Unamortized Gain (Loss) [Member] | Other Postretirement Benefits Plan [Member]        
Accumulated Other Comprehensive (Income) Loss, Defined Benefit Plan, after Tax 2.5 2.7 2.5 2.7
Reclassification from Accumulated Other Comprehensive Income, Current Period, Net of Tax 0.0 0.0    
Accumulated Defined Benefit Plans Adjustment, Net Prior Service Cost (Credit) [Member] | Pension Plan [Member]        
Accumulated Other Comprehensive (Income) Loss, Defined Benefit Plan, after Tax 0.0 0.1 0.0 0.1
Reclassification from Accumulated Other Comprehensive Income, Current Period, Net of Tax 0.0 0.0    
Accumulated Defined Benefit Plans Adjustment, Net Prior Service Cost (Credit) [Member] | Other Postretirement Benefits Plan [Member]        
Accumulated Other Comprehensive (Income) Loss, Defined Benefit Plan, after Tax 6.5 0.0 $ 7.0 $ 0.0
Reclassification from Accumulated Other Comprehensive Income, Current Period, Net of Tax $ (0.5) $ 0.0