Note 8 - Revenue - Domestic Revenues an Export Revenues (Details) $ in Thousands, T in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2020 USD ($) T	Sep. 30, 2019 USD ($)	Sep. 30, 2020 USD ($) T $ / T	Sep. 30, 2019 USD ($)
REVENUES
Revenues | $	$ 39,459	$ 61,380	$ 117,769	$ 184,601
Term of domestic sales contracts			1 year
Domestic Coal Revenues
REVENUES
Revenues | $	13,546	50,382	$ 64,764	126,498
Export Revenues
REVENUES
Revenues | $	$ 25,913	$ 10,998	$ 53,005	$ 58,103
Fixed Priced Contracts
REVENUES
Outstanding performance obligation, mass | T	0.6		0.6
Average per ton | $ / T			88
Fixed Priced Contracts | Plan
REVENUES
Outstanding performance obligation, mass | T	0.2		0.2
Average per ton | $ / T			91
Contracts with Indexed Based Pricing Mechanisms
REVENUES
Outstanding performance obligation, mass | T	0.2		0.2