Note 14 - Commitments and Contingent Liabilities (Details Textual) $ in Millions	Aug. 23, 2017	Apr. 24, 2017	Mar. 31, 2021 USD ($)	Dec. 31, 2020 USD ($)
Number of Mines Sold			5
Guarantees, Maximum Exposure			$ 7	$ 8
Casey Litigation [Member] | Pending Litigation [Member]
Loss Contingency, Number of Plaintiffs	2
United Mine Workers of America 1992 Benefit Plan Litigation [Member] | Pending Litigation [Member] | Minimum [Member]
Estimated Liability, Annual Servicing Cost, Next Ten Years			10
United Mine Workers of America 1992 Benefit Plan Litigation [Member] | Pending Litigation [Member] | Maximum [Member]
Estimated Liability, Annual Servicing Cost, Next Ten Years			$ 20
Fitzwater Litigation [Member] | Pending Litigation [Member]
Loss Contingency, Number of Plaintiffs		3