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Inventories
3 Months Ended
Mar. 31, 2018
Inventory Disclosure [Abstract]  
Inventories
Inventories

Inventories are summarized as follows:
 
MARCH 31
2018
 
DECEMBER 31
2017
Coal
$
12,371

 
$
13,416

Mining supplies
17,744

 
16,599

 Total inventories
$
30,115

 
$
30,015