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Accumulated Other Comprehensive Loss (Details) - USD ($)
$ in Millions
 12 Months Ended
Dec. 31, 2020
Dec. 31, 2019
Dec. 31, 2018
Reclassification Out of Accumulated Other Comprehensive Income [Line Items]      
Accumulated other comprehensive loss, net of tax $ (54.3) $ (59.5)  
Net current period other comprehensive income ending balance 5.2 7.8 $ (10.5)
Other Comprehensive (Income) Loss, Defined Benefit Plan, Reclassification Adjustment from AOCI, Tax (2.5) (1.9) (2.4)
Defined Benefit Pension Plan Adjustments      
Reclassification Out of Accumulated Other Comprehensive Income [Line Items]      
Accumulated other comprehensive loss, net of tax (54.3) (59.5) (67.3)
Other Comprehensive Income (Loss), before Reclassifications, Net of Tax (2.1) 2.7  
Reclassification from Accumulated Other Comprehensive Income, Current Period, Net of Tax 7.3 5.1  
Net current period other comprehensive income ending balance   7.8  
Pension Benefit Plans | Defined Benefit Pension Plan Adjustments      
Reclassification Out of Accumulated Other Comprehensive Income [Line Items]      
Accumulated other comprehensive loss, net of tax (54.5) (67.8) (83.0)
Other Comprehensive Income (Loss), before Reclassifications, Net of Tax 6.0 9.8  
Reclassification from Accumulated Other Comprehensive Income, Current Period, Net of Tax 7.3 5.4  
Net current period other comprehensive income ending balance 13.3 15.2  
Other Postretirement Benefits Plan [Member]      
Reclassification Out of Accumulated Other Comprehensive Income [Line Items]      
Accumulated other comprehensive loss, net of tax 0.2 8.3 $ 15.7
Other Comprehensive Income (Loss), before Reclassifications, Net of Tax (8.1) (7.1)  
Reclassification from Accumulated Other Comprehensive Income, Current Period, Net of Tax 0.0 (0.3)  
Net current period other comprehensive income ending balance $ (8.1) $ (7.4)