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Inventories (Tables)
6 Months Ended
Jun. 30, 2021
Inventory Disclosure [Abstract]  
Schedule of Inventories
Inventories are summarized as follows:
 JUNE 30
2021
 DECEMBER 31
2020
Coal$16,025 $17,695 
Mining supplies33,691 29,856 
 Total inventories$49,716  $47,551