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Pension and Postretirement Benefit Plans (Net Periodic Pension Costs Included in Accumulated Other Comprehensive Income (Loss)) (Details) - USD ($)
$ in Millions
12 Months Ended
Dec. 31, 2023
Dec. 31, 2022
Dec. 31, 2021
After-Tax Amount      
Prior service (cost) credit arising during the year $ 0.0 $ (19.1) $ 10.0
Net loss (gain) recognized due to settlement 0.4 (0.4) 0.1
Net actuarial gain (loss) arising during the year (16.1) 12.3 53.6
Amortization of prior service cost 1.3 0.0 0.6
Amortization of net actuarial losses 7.0 6.4 12.3
Defined Benefit Pension Plans      
After-Tax Amount      
Prior service (cost) credit arising during the year   (19.1) 10.0
Net loss (gain) recognized due to settlement 0.4 (0.4) 0.1
Net actuarial gain (loss) arising during the year (16.1) 12.3 53.6
Amortization of prior service cost 1.3   0.6
Amortization of net actuarial losses 7.0 6.4 12.3
Postretirement benefits | Defined Benefit Pension Plans      
Before-Tax Amount      
Accumulated other comprehensive loss, beginning of period (304.4) (302.4)  
Prior service (cost) credit arising during the year 0.0 (25.5)  
Net (gain) loss recognized due to settlement 0.4 (0.4)  
Net actuarial gain (loss) arising during the year (21.5) 15.0  
Amortization of prior service cost 1.7 0.2  
Amortization of net actuarial losses 9.4 8.7  
Accumulated other comprehensive loss, end of period (314.4) (304.4) (302.4)
Income Tax      
Accumulated other comprehensive loss, beginning of period (73.2) (72.0)  
Prior service (cost) credit arising during the year 0.0 (6.4)  
Net (gain) loss recognized due to settlement 0.0 0.0  
Net actuarial gain (loss) arising during the year (5.4) 2.7  
Amortization of prior service cost 0.4 0.2  
Amortization of net actuarial losses 2.4 2.3  
Accumulated other comprehensive loss, end of period (75.8) (73.2) (72.0)
After-Tax Amount      
Accumulated other comprehensive loss, beginning of period (231.2) (230.4)  
Prior service (cost) credit arising during the year 0.0 (19.1)  
Net loss (gain) recognized due to settlement 0.4 (0.4)  
Net actuarial gain (loss) arising during the year (16.1) 12.3  
Amortization of prior service cost 1.3 0.0  
Amortization of net actuarial losses 7.0 6.4  
Accumulated other comprehensive loss, end of period $ (238.6) $ (231.2) $ (230.4)