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Derivatives and Fair Value Measurements- Changes in Recurring Level 3 Net Financial Assets (Details) - Coal Trading - USD ($)
$ in Millions
12 Months Ended
Dec. 31, 2023
Dec. 31, 2022
Dec. 31, 2021
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]      
Beginning of period $ 2.5 $ 4.0 $ 4.0
Impairment loss included in earnings (2.0) (1.7) 0.0
Purchases 0.0 0.2 0.0
Exchange (0.5) 0.0 0.0
End of period $ 0.0 $ 2.5 $ 4.0