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Income Taxes - Tax Effects of Temporary Differences on Deferred Tax Assets and Liabilities (Details) - USD ($)
$ in Millions
Dec. 31, 2025
Dec. 31, 2024
Deferred tax assets:    
Tax loss carryforwards and credits $ 808.2 $ 732.5
Property, plant, equipment and mine development, principally due to differences in depreciation, depletion and asset impairments 539.0 523.2
Accrued postretirement benefit obligations 29.3 33.7
Asset retirement obligations 108.8 102.4
Employee benefits 19.3 19.7
Take-or-pay obligations 5.9 5.5
Investments and other assets 28.6 32.6
Workers’ compensation obligations 9.3 8.4
Operating lease liabilities 29.5 29.0
Other 38.6 33.0
Total gross deferred tax assets 1,616.5 1,520.0
Deferred Tax Assets, Valuation Allowance (1,469.2) (1,420.9)
Total net deferred tax assets 147.3 99.1
Deferred tax liabilities:    
Property, plant, equipment and mine development, principally due to differences in depreciation, depletion and asset impairments 123.7 95.5
Operating lease right-of-use assets 28.9 29.1
Investments and other assets 18.8 15.4
Total deferred tax liabilities 171.4 140.0
Deferred taxes are classified as follows on the Consolidated Balance Sheets:    
Noncurrent deferred income tax asset 2.2 0.0
Noncurrent deferred income tax liability 26.3 40.9
Net Deferred Tax Liabilities $ 24.1 $ 40.9