Income Taxes Schedule of Significant Components of Deferred Tax Assets and Liabilities (Details) - USD ($)	Dec. 31, 2018	Dec. 31, 2017
Deferred Tax Asset:
Postretirement benefits other than pensions	$ 108,603,000	$ 131,354,000
Asset retirement obligations	57,956,000	51,415,000
Pneumoconiosis benefits	41,632,000	36,160,000
Workers' compensation	16,016,000	16,778,000
Salary retirement	15,855,000	12,465,000
Mine subsidence	15,097,000	15,322,000
Financing	9,387,000	0
State bonus, net of Federal	6,042,000	4,473,000
Long-term disability	2,798,000	3,375,000
Other	6,669,000	7,924,000
Total Deferred Tax Asset	280,055,000	279,266,000
Valuation Allowance	0	(1,379,000)
Net Deferred Tax Asset	280,055,000	277,887,000
Deferred Tax Liability:
Property, plant and equipment	(175,558,000)	(174,806,000)
Equity Partnerships	(16,638,000)	(17,991,000)
Advance mining royalties	(10,314,000)	(10,025,000)
Total Deferred Tax Liability	(202,510,000)	(202,822,000)
Net Deferred Tax Asset	$ 77,545,000	$ 75,065,000