Current and Long-Term Financing (Narrative) (Details) - USD ($)	12 Months Ended
	Dec. 31, 2020	Dec. 31, 2019	Dec. 15, 2026
Line of Credit Facility [Line Items]
Interest paid	$ 1,400,000	$ 900,000
Outstanding notes payable	44,997,000	24,300,000
NACoal
Line of Credit Facility [Line Items]
Line of credit facility, maximum borrowing capacity	150,000,000.0
Line of credit facility, amount outstanding	30,000,000	16,000,000
Line of credit facility, remaining borrowing capacity	116,951,000	132,644,000
Amount of letters of credit outstanding	$ 3,000,000.0
Line of credit facility, unused capacity, commitment fee percentage	0.30%
Weighted average interest rate	1.88%
Minimum interest coverage ratio	4.00
Fixed charge coverage ratio	1.10
Line of credit facility, availability required to pay dividends	$ 15,000,000.0
NACoal | Maximum
Line of Credit Facility [Line Items]
Maximum debt to EBITDA ratio	3.00
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]
Outstanding notes payable	$ 4,400,000	$ 2,000,000.0
Interest rate	0.14%	1.68%
Secured Debt | Notes Payable, Maturing December 2026 And April 1, 2027
Line of Credit Facility [Line Items]
Outstanding notes payable	$ 10,600,000	$ 6,300,000
Interest rate	4.20%
Secured Debt | Notes Payable, Maturing December 2026
Line of Credit Facility [Line Items]
Debt instrument term	10 years
Secured Debt | Notes Payable, Maturing April 1, 2027
Line of Credit Facility [Line Items]
Debt instrument term	7 years
Secured Debt | Forecast | Notes Payable, Maturing December 2026
Line of Credit Facility [Line Items]
Principal payment due at maturity			$ 4,400,000