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Income Taxes - Tax Effects of Temporary Differences on Deferred Tax Assets and Liabilities (Details) - USD ($)
$ in Millions
Dec. 31, 2023
Dec. 31, 2022
Deferred tax assets:    
Tax loss carryforwards and credits $ 745.6 $ 740.1
Property, plant, equipment and mine development, principally due to differences in depreciation, depletion and asset impairments 550.1 550.8
Accrued postretirement benefit obligations 38.6 41.4
Asset retirement obligations 99.1 95.1
Employee benefits 20.1 22.1
Take-or-pay obligations 6.7 8.2
Hedge activities 0.0 49.1
Investments and other assets 40.8 37.0
Workers’ compensation obligations 7.3 7.1
Operating lease liabilities 17.2 7.8
Other 30.6 28.3
Total gross deferred tax assets 1,556.1 1,587.0
Valuation allowance, income tax (1,473.5) (1,451.0)
Total deferred tax assets 82.6 136.0
Deferred tax liabilities:    
Property, plant, equipment and mine development, principally due to differences in depreciation, depletion and asset impairments 84.9 67.5
Operating lease right-of-use assets 16.3 7.6
Investments and other assets 10.0 6.6
Total deferred tax liabilities 111.2 81.7
Deferred taxes are classified as follows:    
Noncurrent deferred income tax asset 0.0 74.7
Noncurrent deferred income tax liability 28.6 20.4
Net Deferred Tax Liabilities $ 28.6  
Net Deferred Tax Assets   $ 54.3