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Income Taxes - Significant Components of our deferred tax assets & liabilities (Details) - USD ($)
$ in Millions
Dec. 31, 2024
Dec. 31, 2023
Deferred tax assets:    
Accrued postretirement benefit cost $ 30.1 $ 30.7
Net operating losses 473.3 467.6
Disallowed interest expense 37.1 29.2
Derivative and hedging contracts 0.1 1.2
Fixed asset tax over book basis 0.0 9.2
Other 29.4 28.3
Total deferred tax assets 570.0 566.2
Valuation allowance (504.4) (537.6)
Net deferred tax assets 65.6 28.6
Deferred tax liabilities:    
Fixed asset book over tax basis (115.9) (62.0)
Derivatives 0.0 0.0
Foreign basis differences 0.6 (18.1)
Other (21.4) (20.6)
Total deferred tax liabilities (136.7) (100.7)
Net deferred tax liability $ (71.1) $ (72.1)