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Intangible Assets (Details) - USD ($)
$ in Thousands
12 Months Ended
Dec. 31, 2019
Dec. 31, 2018
Finite-Lived Intangible Assets, Net [Abstract]    
Net Balance $ 37,902 $ 40,516
Amortization of intangible assets 2,614 3,038
Coal supply agreement    
Finite-Lived Intangible Assets, Net [Abstract]    
Gross Carrying Amount 84,200 84,200
Accumulated Amortization (46,298) (43,684)
Net Balance 37,902 $ 40,516
NACoal    
Finite-Lived Intangible Assets, Net [Abstract]    
Expected annual amortization expense, 2020 3,000  
Expected annual amortization expense, 2021 3,100  
Expected annual amortization expense, 2022 3,100  
Expected annual amortization expense, 2023 3,100  
Expected annual amortization expense, 2024 $ 3,100  
Coal supply agreement amortization period 30 years