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Pension Benefits - Changes in Fair Value of Plan Assets and Changes in Accumulated Benefit Obligations (Details) - Pension Plans, Defined Benefit - USD ($)
$ in Millions
 12 Months Ended
Dec. 31, 2021
Dec. 31, 2020
Dec. 31, 2019
Defined Benefit Plan, Change in Fair Value of Plan Assets [Roll Forward]      
Fair Value of Plans’ Assets at Beginning of Year $ 405.4 $ 664.6  
Actual Return on Plan Assets (0.7) 92.1  
Employer Contributions 0.0 0.0  
Benefits Paid (13.0) (145.9)  
Settlement Benefits 0.0 (205.4)  
Benefits Paid (13.0) (145.9)  
Fair Value of Plan Assets at End of Year 391.7 405.4 $ 664.6
Defined Benefit Plan, Change in Benefit Obligation [Roll Forward]      
Accumulated Postretirement Benefit Obligation at Beginning of Year 382.3 660.5  
Interest Cost 7.2 16.5 22.3
Benefits Paid (13.0) (145.9)  
Settlement Benefits 0.0 (205.4)  
Plan Amendments (18.3) 0.0  
Actuarial (Gains) Losses (16.0) 56.6  
Projected Benefit Obligation at End of Year 378.8 382.3 $ 660.5
Funded Status—Plan Assets in Excess (Deficit) of Projected Benefit Obligation 12.9 23.1  
Unamortized Amount Reported in AOCI at End of Year (77.6) (68.2)  
Accumulated Benefit Obligation at End of Year $ 378.8 $ 382.3