Note 8 - Revenue - Domestic Revenues an Export Revenues (Details) $ in Thousands, T in Millions	3 Months Ended		6 Months Ended
	Jun. 30, 2020 USD ($) T	Jun. 30, 2019 USD ($)	Jun. 30, 2020 USD ($) T $ / T	Jun. 30, 2019 USD ($)
REVENUES
Revenues | $	$ 36,374	$ 65,761	$ 78,310	$ 123,221
Term of domestic sales contracts			1 year
Domestic Coal Revenues
REVENUES
Revenues | $	20,686	39,544	$ 51,218	76,116
Export Revenues
REVENUES
Revenues | $	$ 15,688	$ 26,217	$ 27,092	$ 47,105
Fixed Priced Contracts
REVENUES
Outstanding performance obligation, mass | T	0.8		0.8
Average per ton | $ / T			93
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	0.1		0.1