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Current and Long-Term Financing (Narrative) (Details)
 12 Months Ended
Dec. 31, 2019
USD ($)
Dec. 31, 2018
USD ($)
Dec. 15, 2026
USD ($)
Line of Credit Facility [Line Items]      
Interest paid $ 900,000 $ 2,000,000  
Outstanding notes payable 24,300,000 10,600,000  
NACoal      
Line of Credit Facility [Line Items]      
Line of credit facility, maximum borrowing capacity 150,000,000    
Line of credit facility, amount outstanding 16,000,000 4,000,000  
Line of credit facility, remaining borrowing capacity 132,644,000 144,481,000  
Amount of letters of credit outstanding $ 1,400,000    
Line of credit facility, unused capacity, commitment fee percentage 0.30%    
Weighted average interest rate 5.50%    
Minimum interest coverage ratio 4.00    
Fixed charge coverage ratio 1.10    
Line of credit facility, availability required to pay dividends $ 15,000,000    
NACoal | Maximum      
Line of Credit Facility [Line Items]      
Maximum EBITDA ratio 3.00    
NACoal | Minimum      
Line of Credit Facility [Line Items]      
Maximum EBITDA ratio 2.00    
NACoal | Base Rate      
Line of Credit Facility [Line Items]      
Basis spread on variable rate 0.75%    
NACoal | LIBOR Rate      
Line of Credit Facility [Line Items]      
Basis spread on variable rate 1.75%    
Secured Debt | Demand Note Payable to Unconsolidated Subsidiary      
Line of Credit Facility [Line Items]      
Interest rate 1.68%    
Outstanding notes payable $ 2,000,000    
Secured Debt | 5.29% Note Payable Maturing 2026 [Member]      
Line of Credit Facility [Line Items]      
Interest rate 5.29%    
Debt instrument term 10 years    
Outstanding notes payable $ 6,300,000 $ 6,600,000  
Secured Debt | Forecast | 5.29% Note Payable Maturing 2026 [Member]      
Line of Credit Facility [Line Items]      
Principal payment due at maturity     $ 4,400,000