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Pension and Other Postretirement Benefits - Net Periodic Benefit Cost (Details) - USD ($)
$ in Millions
 12 Months Ended
Dec. 31, 2023
Dec. 31, 2022
Dec. 31, 2021
Pension      
Defined Benefit Plan Disclosure [Line Items]      
Service cost $ 2.4 $ 4.3 $ 4.7
Interest cost 14.0 10.3 9.6
Expected return on plan assets (15.1) (23.5) (22.4)
Amortization of prior service costs 0.1 0.1 0.1
Amortization of net loss 6.0 3.5 6.1
Net periodic benefit cost 7.4 (5.3) (1.9)
Curtailment benefit 0.0 0.0 0.0
Total benefit cost 7.4 (5.3) (1.9)
OPEB      
Defined Benefit Plan Disclosure [Line Items]      
Service cost 0.1 0.2 0.2
Interest cost 3.8 2.9 2.4
Expected return on plan assets 0.0 0.0 0.0
Amortization of prior service costs 0.0 (1.3) (3.2)
Amortization of net loss 0.2 1.3 2.3
Net periodic benefit cost 4.1 3.1 1.7
Curtailment benefit 0.0 (8.9) 0.0
Total benefit cost $ 4.1 $ (5.8) $ 1.7