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Intangible Assets (Tables)
 12 Months Ended
Dec. 31, 2018
Goodwill and Intangible Assets Disclosure [Abstract]  
Schedule of Finite-Lived Intangible Assets
Intangible assets other than goodwill, which are subject to amortization, consist of the following:
 
Gross Carrying
Amount
 
Accumulated
Amortization
 
Net
Balance
Balance at December 31, 2018
 
 
 
 
 
Coal supply agreement
$
84,200

 
$
(43,684
)
 
$
40,516

 
 
 
 
 
 
Balance at December 31, 2017
 
 
 
 
 
Coal supply agreement
$
84,200

 
$
(40,646
)
 
$
43,554