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Inventories
12 Months Ended
Dec. 31, 2017
Inventory Disclosure [Abstract]  
Inventories
Inventories

Inventories are summarized as follows:
 
December 31
 
2017
 
2016
Coal
$
13,416

 
$
13,137

Mining supplies
16,599

 
15,790

Total inventories
$
30,015

 
$
28,927