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Subsequent Events (Details) - USD ($)
$ in Thousands
Oct. 11, 2018
Sep. 30, 2018
Aug. 07, 2018
Dec. 31, 2017
Subsequent Event [Line Items]        
Current assets held for sale   $ 255,519   $ 0
Non-Core Assets Powder River Basin [Member] | Disposal Group, Held-for-sale, Not Discontinued Operations        
Subsequent Event [Line Items]        
Option payment on assets held for sale     $ 30,000  
Current assets held for sale   255,500    
Accounts receivables of assets held for sale   1,100    
Current liabilities of assets held for sale   $ 2,700    
Non-Core Assets Powder River Basin [Member] | Disposal Group, Disposed of by Sale, Not Discontinued Operations [Member]        
Subsequent Event [Line Items]        
Option payment on assets held for sale     $ 30,000  
Non-Core Assets Powder River Basin [Member] | Disposal Group, Disposed of by Sale, Not Discontinued Operations [Member] | Subsequent Event        
Subsequent Event [Line Items]        
Net proceeds received on divestiture of assets held for sale $ 300,000