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Income Taxes - Significant Components of our deferred tax assets & liabilities (Details) - USD ($)
$ in Millions
Dec. 31, 2023
Dec. 31, 2022
Deferred tax assets:    
Accrued postretirement benefit cost $ 30.7 $ 25.7
Net operating losses 467.6 395.8
Disallowed interest expense 29.2 27.7
Derivative and hedging contracts 1.2 0.0
Fixed asset tax over book basis 9.2 17.0
Other 28.3 26.1
Total deferred tax assets 566.2 492.3
Valuation allowance (537.6) (487.9)
Net deferred tax assets 28.6 4.4
Deferred tax liabilities:    
Fixed asset book over tax basis (62.0) (60.5)
Derivatives 0.0 (19.0)
Foreign basis differences (18.1) (19.8)
Other (20.6) (7.9)
Total deferred tax liabilities (100.7) (107.2)
Net deferred tax liability $ (72.1) $ (102.8)