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Inventories (Tables)
6 Months Ended
Jun. 30, 2018
Inventory Disclosure [Abstract]  
Schedule of Inventories
Inventories are summarized as follows:
 
JUNE 30
2018
 
DECEMBER 31
2017
Coal
$
11,434

 
$
13,416

Mining supplies
17,725

 
16,599

 Total inventories
$
29,159

 
$
30,015