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Gain on Property Insurance Recovery Related to Mountain Laurel Longwall (Details)
$ in Thousands
9 Months Ended
Sep. 30, 2020
USD ($)
Jun. 30, 2020
item
Business Interruption Loss [Line Items]    
Gain on Business Interruption Insurance Recovery | $ $ 23,518  
Longwall system's not recoverable | item   123
Mountain Laurel Longwall Operations    
Business Interruption Loss [Line Items]    
Gain on Business Interruption Insurance Recovery | $ $ 23,500  
Hydraulic Shields not recoverable | item   176