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Income Taxes Tax Effects of Temorpary Differences on Deferred Tax Assets and Liabilities (Details) - USD ($)
$ in Millions
Dec. 31, 2017
Dec. 31, 2016
Successor    
Tax Effects of Temporary Differences on Deferred Tax Assets and Liabilities [Line Items]    
Deferred Income Taxes and Other Assets, Noncurrent $ 85.6  
Deferred tax assets:    
Deferred Tax Assets, Operating Loss Carryforwards 2,068.0  
Deferred Tax Assets, Property, Plant and Equipment 463.8  
Accrued postretirement benefit obligations 194.2  
Asset retirement obligations 30.6  
Deferred Tax Assets Financial Guarantee 2.0  
Deferred Tax Assets, Tax Deferred Expense, Compensation and Benefits, Employee Benefits 25.3  
Deferred Tax Asset, Payments to the VEBA to settle the accumulated postretirement healthcare benefit obligations 27.2  
Deferred Tax Assets, Investments 10.5  
Deferred Tax Assets, Derivative Instruments 137.2  
Workers’ compensation obligations 6.4  
Other 16.1  
Total gross deferred tax assets 2,981.3  
Valuation allowance, income tax (2,432.5)  
Deferred Tax Assets, Net of Valuation Allowance 548.8  
Deferred tax liabilities:    
Property, plant, equipment and mine development, principally due to differences in depreciation, depletion and asset impairments 353.3  
Unamortized discount on Convertible Junior Subordinated Debentures 0.0  
Deferred Tax Liabilities Coal Supply Agreements 29.6  
Investments and other assets 85.7  
Deferred Tax Liabilities, Gross 468.6  
Deferred taxes are classified as follows:    
Deferred Income Taxes and Other Tax Liabilities, Noncurrent (5.4)  
Deferred Tax Assets, Net $ (80.2)  
Predecessor    
Tax Effects of Temporary Differences on Deferred Tax Assets and Liabilities [Line Items]    
Deferred Income Taxes and Other Assets, Noncurrent   $ 0.0
Deferred tax assets:    
Deferred Tax Assets, Operating Loss Carryforwards   4,284.4
Deferred Tax Assets, Property, Plant and Equipment   424.4
Accrued postretirement benefit obligations   364.5
Asset retirement obligations   163.6
Deferred Tax Assets Financial Guarantee   77.9
Deferred Tax Assets, Tax Deferred Expense, Compensation and Benefits, Employee Benefits   57.0
Deferred Tax Asset, Payments to the VEBA to settle the accumulated postretirement healthcare benefit obligations   0.0
Deferred Tax Assets, Investments   21.0
Deferred Tax Assets, Derivative Instruments   0.0
Workers’ compensation obligations   7.5
Other   2.1
Total gross deferred tax assets   5,402.4
Valuation allowance, income tax   (4,037.5)
Deferred Tax Assets, Net of Valuation Allowance   1,364.9
Deferred tax liabilities:    
Property, plant, equipment and mine development, principally due to differences in depreciation, depletion and asset impairments   1,324.8
Unamortized discount on Convertible Junior Subordinated Debentures   127.7
Deferred Tax Liabilities Coal Supply Agreements   0.0
Investments and other assets   86.3
Deferred Tax Liabilities, Gross   1,538.8
Deferred taxes are classified as follows:    
Deferred Income Taxes and Other Tax Liabilities, Noncurrent   (173.9)
Deferred Tax Liabilities, Net   $ (173.9)