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Inventories (Tables)
9 Months Ended
Sep. 30, 2018
Inventory Disclosure [Abstract]  
Schedule of Inventories
Inventories are summarized as follows:
 
SEPTEMBER 30
2018
 
DECEMBER 31
2017
Coal
$
13,484

 
$
13,416

Mining supplies
19,698

 
16,599

 Total inventories
$
33,182

 
$
30,015