Note 6 - Income Taxes - Schedule of Significant Components of Deferred Tax Assets and Liabilities (Details) - USD ($) $ in Thousands	Dec. 31, 2020	Dec. 31, 2019
Deferred Tax Asset:
Postretirement benefits other than pensions	$ 101,673	$ 110,504
Pneumoconiosis benefits	60,284	52,521
Asset retirement obligations	56,779	60,260
Workers' compensation	17,493	16,750
Mine subsidence	17,271	17,110
Operating lease liability	11,377	14,757
Salary retirement	9,446	14,761
State bonus, net of Federal	6,918	7,042
Net operating loss	6,134	0
Compensation	5,158	3,841
Long-term disability	2,757	3,031
Financing	2,077	16,806
Foreign tax credits	0	1,400
Other	4,175	2,456
Total Deferred Tax Asset	301,542	321,239
Valuation Allowance	(2,879)	(1,400)
Net Deferred Tax Asset	298,663	319,839
Deferred Tax Liability:
Property, plant and equipment	(172,026)	(173,849)
Equity Partnerships	(35,570)	(17,028)
Right of use assets	(11,338)	(14,757)
Advance mining royalties	(10,908)	(10,700)
Total Deferred Tax Liability	(229,842)	(216,334)
Net Deferred Tax Asset	$ 68,821	$ 103,505