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Postretirement Health Care and Life Insurance Benefits - Net Periodic Postretirement Benefit Cost (Details) - Postretirement Health Care and Life Insurance Benefits - USD ($)
$ in Millions
 12 Months Ended
Dec. 31, 2023
Dec. 31, 2022
Dec. 31, 2021
Defined Benefit Plan Disclosure [Line Items]      
Service cost for benefits earned $ 0.5 $ 0.8 $ 1.0
Interest cost on accumulated postretirement benefit obligation 10.2 7.0 10.5
Expected return on plan assets (0.5) (0.8) (1.0)
Amortization of prior service credit (53.8) (53.8) (46.4)
Net actuarial gain (2.6) (51.2) (54.5)
Net periodic postretirement benefit credit $ (46.2) $ (98.0) $ (90.4)