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Income Taxes - Tax Effects of Temporary Differences on Deferred Tax Assets and Liabilities (Details) - USD ($)
$ in Millions
Dec. 31, 2024
Dec. 31, 2023
Deferred tax assets:    
Tax loss carryforwards and credits $ 732.5 $ 745.6
Property, plant, equipment and mine development, principally due to differences in depreciation, depletion and asset impairments 523.2 550.1
Accrued postretirement benefit obligations 33.7 38.6
Asset retirement obligations 102.4 99.1
Employee benefits 19.7 20.1
Take-or-pay obligations 5.5 6.7
Investments and other assets 32.6 40.8
Workers’ compensation obligations 8.4 7.3
Operating lease liabilities 29.0 17.2
Other 33.0 30.6
Total gross deferred tax assets 1,520.0 1,556.1
Valuation allowance (1,420.9) (1,473.5)
Total net deferred tax assets 99.1 82.6
Deferred tax liabilities:    
Property, plant, equipment and mine development, principally due to differences in depreciation, depletion and asset impairments 95.5 84.9
Operating lease right-of-use assets 29.1 16.3
Investments and other assets 15.4 10.0
Total deferred tax liabilities 140.0 111.2
Deferred taxes are classified as follows:    
Noncurrent deferred income tax liability 40.9 28.6
Net Deferred Tax Liabilities $ 40.9 $ 28.6