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Commitments and Contingencies (Details) - USD ($)
3 Months Ended 6 Months Ended 9 Months Ended
Sep. 30, 2017
Sep. 30, 2016
Mar. 31, 2016
Sep. 30, 2016
Sep. 30, 2017
Dec. 31, 2016
Successor            
Guarantor Obligations [Line Items]            
Throughput obligation $ 0       $ 0  
Future minimum operating lease payments 0       0 $ 0
Coal royalty expense 22,300,000 $ 2,700,000   $ 6,700,000 74,500,000  
Successor | Indemnifications            
Guarantor Obligations [Line Items]            
Remaining maximum exposure 200,000       200,000  
Successor | Other long-term liabilities | Indemnifications            
Guarantor Obligations [Line Items]            
Indemnification accrual $ 300,000       $ 300,000 300,000
Successor | Accrued expenses            
Guarantor Obligations [Line Items]            
Throughput obligation           $ 2,100,000
Predecessor            
Guarantor Obligations [Line Items]            
Coal royalty expense     $ 3,600,000