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Components of Accumulated Other Comprehensive Loss - Components of AOCI (Details) - USD ($)
$ in Millions
 3 Months Ended 9 Months Ended
Sep. 30, 2021
Sep. 30, 2020
Sep. 30, 2021
Sep. 30, 2020
Dec. 31, 2020
Accumulated Other Comprehensive Income (Loss) [Line Items]          
Other comprehensive loss before income tax effect $ (117.7)   $ (117.7)   $ (121.6)
Income tax effect 2.5   2.5   2.8
Accumulated other comprehensive loss (115.2)   (115.2)   (118.8)
AOCI Attributable to Parent, Net of Tax [Roll Forward]          
Balance, start 370.4 $ 648.7 546.1 $ 675.0  
Net amount reclassified to net income (loss) 1.2 1.1 3.6 3.3  
Balance, end 320.2 592.4 320.2 592.4  
Defined benefit plan and other postretirement liabilities [Member]          
Accumulated Other Comprehensive Income (Loss) [Line Items]          
Other comprehensive loss before income tax effect (119.7)   (119.7)   (123.7)
Income tax effect 2.9   2.9   3.2
AOCI Attributable to Parent, Net of Tax [Roll Forward]          
Balance, start (118.1) (109.5) (120.6) (111.7)  
Net amount reclassified to net income (loss) 1.2 1.2 3.7 3.4  
Balance, end (116.9) (108.3) (116.9) (108.3)  
Unrealized gain (loss) on financial instruments [Member]          
Accumulated Other Comprehensive Income (Loss) [Line Items]          
Other comprehensive loss before income tax effect 2.0   2.0   2.1
Income tax effect (0.4)   (0.4)   $ (0.4)
AOCI Attributable to Parent, Net of Tax [Roll Forward]          
Balance, start 1.7 1.9 1.8 1.9  
Net amount reclassified to net income (loss) (0.0) (0.1) (0.1) (0.1)  
Balance, end 1.7 1.8 1.7 1.8  
Accumulated other comprehensive loss [Member]          
AOCI Attributable to Parent, Net of Tax [Roll Forward]          
Balance, start (116.4) (107.6) (118.8) (109.8)  
Balance, end $ (115.2) $ (106.5) $ (115.2) $ (106.5)