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Pension and Other Postretirement Benefits - Changes in Plan Assets & Benefit Obligation Recognized in OCI (pre-taxed) (Details) - USD ($)
$ in Millions
 12 Months Ended
Dec. 31, 2023
Dec. 31, 2022
Dec. 31, 2021
Defined Benefit Plan Disclosure [Line Items]      
Net loss (gain) $ 10.1 $ 5.9 $ (31.6)
Amortization of prior service (cost) benefit, including curtailment (0.1) 1.2 3.1
Pension      
Defined Benefit Plan Disclosure [Line Items]      
Net loss (gain) 4.4 29.0  
Prior service cost (benefit) 1.2 0.0  
Amortization of net loss, including recognition due to settlement (6.0) (3.5)  
Amortization of prior service (cost) benefit, including curtailment (0.1) (0.1)  
Total amount recognized in other comprehensive loss (0.5) 25.4  
Total benefit cost 7.4 (5.3) (1.9)
Total recognized in net periodic benefit cost and other comprehensive loss 6.9 20.1  
OPEB      
Defined Benefit Plan Disclosure [Line Items]      
Net loss (gain) 4.5 (23.1)  
Prior service cost (benefit) 0.0 0.0  
Amortization of net loss, including recognition due to settlement (0.2) (1.4)  
Amortization of prior service (cost) benefit, including curtailment 0.0 10.3  
Total amount recognized in other comprehensive loss 4.3 (14.2)  
Total benefit cost 4.1 (5.8) $ 1.7
Total recognized in net periodic benefit cost and other comprehensive loss $ 8.4 $ (20.0)