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Divestitures (Details)
$ in Thousands
3 Months Ended 12 Months Ended
Sep. 14, 2017
USD ($)
company
Dec. 31, 2016
USD ($)
Dec. 31, 2018
USD ($)
Dec. 31, 2017
USD ($)
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]        
Gain on sale of Lone Mountain Processing, Inc.   $ 0 $ 0 $ 21,297
Lone Mountain Processing LLC | Discontinued operations, disposed of by sale        
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]        
Number of mining companies | company 2      
Proceeds from sale $ 8,300      
Disbursements related to landholder consent fees and professional fees 1,400      
Gain on sale of Lone Mountain Processing, Inc. 21,300      
Pension Benefits        
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]        
Curtailment gains   $ 0 $ 0 $ 0
Pension Benefits | Lone Mountain Processing LLC | Discontinued operations, disposed of by sale        
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]        
Curtailment gains $ 4,700