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Inventories (Tables)
12 Months Ended
Dec. 31, 2018
Inventory Disclosure [Abstract]  
Schedule of Inventory
Inventories are summarized as follows:
 
December 31
 
2018
 
2017
Coal
$
11,030

 
$
13,416

Mining supplies
20,179

 
16,599

Total inventories
$
31,209

 
$
30,015