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Inventories (Tables)
3 Months Ended
Mar. 31, 2022
Inventory Disclosure [Abstract]  
Schedule of Inventories
Inventories are summarized as follows:
 MARCH 31
2022
 DECEMBER 31
2021
Coal$18,494 $19,352 
Mining supplies37,473 34,733 
 Total inventories$55,967  $54,085