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Emergence from Bankruptcy and Fresh Start Accounting (Additional Information to Liabilities Subject to Compromise) (Details) - USD ($)
$ in Thousands
Oct. 05, 2016
Oct. 01, 2016
Dec. 31, 2017
Dec. 31, 2016
Fresh-Start Adjustment [Line Items]        
Fair value of senior notes and other long-term debt, including amounts classified as current     $ 336,100 $ 362,900
Current maturities of debt     15,783 11,038
Insurance receivable, current     3,300 $ 800
Decrease in property plant and equipment, net   $ 2,363,829    
Successor        
Fresh-Start Adjustment [Line Items]        
Current maturities of debt     $ 15,783  
Term loan due 2021 | Successor        
Fresh-Start Adjustment [Line Items]        
Fair value of senior notes and other long-term debt, including amounts classified as current   326,500    
Current maturities of debt   $ 3,300    
Minimum | Discounted cash flow valuation technique        
Fresh-Start Adjustment [Line Items]        
Discount rate (percent) 13.25%      
Maximum | Discounted cash flow valuation technique        
Fresh-Start Adjustment [Line Items]        
Discount rate (percent) 15.25%      
Asset Retirement Obligation Costs | Minimum | Discounted cash flow valuation technique        
Fresh-Start Adjustment [Line Items]        
Discount rate (percent)   7.06%    
Asset Retirement Obligation Costs | Maximum | Discounted cash flow valuation technique        
Fresh-Start Adjustment [Line Items]        
Discount rate (percent)   9.08%    
Net Plant & Equipment        
Fresh-Start Adjustment [Line Items]        
Decrease in property plant and equipment, net   $ 405,259    
Net Plant & Equipment | Market Approach and Cost Approach Valuation Techniques | Successor        
Fresh-Start Adjustment [Line Items]        
Fair value of property, plant and equipment, net   407,600    
Net Coal Properties        
Fresh-Start Adjustment [Line Items]        
Decrease in property plant and equipment, net   1,971,314    
Net Coal Properties | Discounted Cash Flow and Market Approach Valuation Techniques | Successor        
Fresh-Start Adjustment [Line Items]        
Fair value of property, plant and equipment, net   387,500    
Fresh Start Adjustments        
Fresh-Start Adjustment [Line Items]        
Increase (decrease) in inventories   (21,078)    
Increase in current above market sales contracts   71,100    
Decrease in prepaid expenses   11,300    
Decrease in property plant and equipment, net   2,363,829    
Increase in noncurrent above market sales contract   26,000    
Increase in noncurrent insurance receivables   18,600    
Decrease in noncurrent prepaid balances   13,200    
Increase in current below market sales contracts   27,800    
Increase in noncurrent below market sales contracts   3,900    
Decrease in miscellaneous noncurrent liabilities   3,100    
Fresh Start Adjustments | Coal Inventory        
Fresh-Start Adjustment [Line Items]        
Increase (decrease) in inventories   7,300    
Fresh Start Adjustments | Critical Spare Parts        
Fresh-Start Adjustment [Line Items]        
Increase (decrease) in inventories   $ (28,400)    
Equity Method Investments | Level 3 | Discounted Cash Flow Approach and Comparable Company Approach        
Fresh-Start Adjustment [Line Items]        
Discount rate (percent)   14.00%