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Inventories (Tables)
3 Months Ended
Mar. 31, 2020
Inventory Disclosure [Abstract]  
Schedule of Inventories
Inventories are summarized as follows:
 
MARCH 31
2020
 
DECEMBER 31
2019
Coal
$
10,521

 
$
15,700

Mining supplies
25,475

 
24,765

 Total inventories
$
35,996

 
$
40,465