Income Taxes (Schedule of Significant Components of Deferred Tax Assets and Liabilities) (Details) - USD ($) $ in Thousands	Dec. 31, 2019	Dec. 31, 2018
Deferred Tax Asset:
Postretirement benefits other than pensions	$ 110,504	$ 108,603
Asset retirement obligations	60,260	57,956
Pneumoconiosis benefits	52,521	41,632
Mine subsidence	17,110	15,097
Financing	16,806	9,387
Workers' compensation	16,750	16,016
Salary retirement	14,761	15,855
Operating lease liabilities	14,757	0
State bonus, net of Federal	7,042	6,042
Long-term disability	3,031	2,798
Foreign tax credits	1,400	0
Other	6,297	6,669
Total Deferred Tax Asset	321,239	280,055
Valuation Allowance	(1,400)	0
Net Deferred Tax Asset	319,839	280,055
Deferred Tax Liability:
Property, plant and equipment	(173,849)	(175,558)
Equity Partnerships	(17,028)	(16,638)
Right of use assets	(14,757)	0
Advance mining royalties	(10,700)	(10,314)
Total Deferred Tax Liability	(216,334)	(202,510)
Net Deferred Tax Asset	$ 103,505	$ 77,545