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Components of Accumulated Other Comprehensive Loss (Details) - USD ($)
$ in Millions
 3 Months Ended 6 Months Ended
Jun. 30, 2021
Jun. 30, 2020
Jun. 30, 2021
Jun. 30, 2020
Dec. 31, 2020
Accumulated Other Comprehensive Income (Loss) [Line Items]          
Other comprehensive loss before income tax effect $ (119.0)   $ (119.0)   $ (121.6)
Income tax effect 2.6   2.6   2.8
Accumulated other comprehensive loss (116.4)   (116.4)   (118.8)
AOCI Attributable to Parent, Net of Tax [Roll Forward]          
Balance, start 408.3 $ 673.8 546.1 $ 675.0  
Net amount reclassified to net income (loss) 1.1 0.8 2.4 2.2  
Balance, end 370.4 648.7 370.4 648.7  
Defined benefit plan and other postretirement liabilities [Member]          
Accumulated Other Comprehensive Income (Loss) [Line Items]          
Other comprehensive loss before income tax effect (121.1)   (121.1)   (123.7)
Income tax effect 3.0   3.0   3.2
AOCI Attributable to Parent, Net of Tax [Roll Forward]          
Balance, start (119.3) (110.3) (120.6) (111.7)  
Net amount reclassified to net income (loss) 1.2 0.8 2.5 2.2  
Balance, end (118.1) (109.5) (118.1) (109.5)  
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   2.1
Income tax effect (0.4)   (0.4)   $ (0.4)
AOCI Attributable to Parent, Net of Tax [Roll Forward]          
Balance, start 1.8 1.9 1.8 1.9  
Net amount reclassified to net income (loss) (0.1) 0.0 (0.1) 0.0  
Balance, end 1.7 1.9 1.7 1.9  
Accumulated other comprehensive loss [Member]          
AOCI Attributable to Parent, Net of Tax [Roll Forward]          
Balance, start (117.5) (108.4) (118.8) (109.8)  
Balance, end $ (116.4) $ (107.6) $ (116.4) $ (107.6)