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Components of Accumulated Other Comprehensive Loss (Details) - USD ($)
$ in Millions
 3 Months Ended
Mar. 31, 2021
Mar. 31, 2020
Dec. 31, 2020
Accumulated Other Comprehensive Income (Loss) [Line Items]      
Other comprehensive loss before income tax effect $ (120.2)   $ (121.6)
Income tax effect 2.7   2.8
Accumulated other comprehensive loss (117.5)   (118.8)
AOCI Attributable to Parent, Net of Tax [Roll Forward]      
Balance, start 546.1 $ 675.0  
Net amount reclassified to net income (loss) 1.3 1.4  
Balance, end 408.3 673.8  
Defined benefit plan and other postretirement liabilities [Member]      
Accumulated Other Comprehensive Income (Loss) [Line Items]      
Other comprehensive loss before income tax effect (122.3)   (123.7)
Income tax effect 3.1   3.2
AOCI Attributable to Parent, Net of Tax [Roll Forward]      
Balance, start (120.6) (111.7)  
Net amount reclassified to net income (loss) 1.3 1.4  
Balance, end (119.3) (110.3)  
Unrealized gain (loss) on financial instruments [Member]      
Accumulated Other Comprehensive Income (Loss) [Line Items]      
Other comprehensive loss before income tax effect 2.1   2.1
Income tax effect (0.4)   $ (0.4)
AOCI Attributable to Parent, Net of Tax [Roll Forward]      
Balance, start 1.8 1.9  
Net amount reclassified to net income (loss) (0.0) 0.0  
Balance, end 1.8 1.9  
Accumulated other comprehensive loss [Member]      
AOCI Attributable to Parent, Net of Tax [Roll Forward]      
Balance, start (118.8) (109.8)  
Balance, end $ (117.5) $ (108.4)