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Income Taxes Tax Effects of Temorpary Differences on Deferred Tax Assets and Liabilities (Details) - Successor - USD ($)
$ in Millions
Dec. 31, 2018
Dec. 31, 2017
Deferred tax assets:    
Tax loss carryforwards and credits $ 1,729.3 $ 2,068.0
Property, plant, equipment and mine development, principally due to differences in depreciation, depletion and asset impairments 304.5 463.8
Accrued postretirement benefit obligations 139.5 194.2
Asset retirement obligations 47.2 30.6
Employee benefits 24.8 25.3
Take or pay obligations 17.1 27.2
Investments and other assets 82.7 137.2
Workers’ compensation obligations 6.2 6.4
Other 38.2 28.6
Total gross deferred tax assets 2,389.5 2,981.3
Valuation allowance, income tax (2,094.3) (2,432.5)
Total deferred tax assets 295.2 548.8
Deferred tax liabilities:    
Property, plant, equipment and mine development, principally due to differences in depreciation, depletion and asset impairments 203.4 353.3
Coal supply agreements 9.3 29.6
Investments and other assets 43.7 85.7
Total deferred tax liabilities 256.4 468.6
Net deferred tax asset 38.8 80.2
Deferred taxes are classified as follows:    
Noncurrent deferred income tax asset 48.5 85.6
Noncurrent deferred income tax liability (9.7) (5.4)
Net deferred tax asset $ 38.8 $ 80.2