Income Taxes Schedule of Significant Components of Deferred Tax Assets and Liabilities (Details) - USD ($) $ in Thousands	Dec. 31, 2017	Dec. 31, 2016
Deferred Tax Asset:
Postretirement benefits other than pensions	$ 131,354	$ 255,507
Asset retirement obligations	51,415	99,467
Pneumoconiosis benefits	36,160	43,371
Workers' compensation	16,778	28,530
Mine subsidence	15,322	39,251
Salary retirement	12,465	37,498
State bonus, net of Federal	4,473	3,175
Long-term disability	3,375	6,358
Other	7,924	8,042
Total Deferred Tax Asset	279,266	521,199
Valuation Allowance	(1,379)	0
Net Deferred Tax Asset	277,887	521,199
Deferred Tax Liability:
Property, plant and equipment	(174,806)	(256,947)
Equity Partnerships	(17,991)	(67,498)
Advance mining royalties	(10,025)	(12,175)
Total Deferred Tax Liability	(202,822)	(336,620)
Net Deferred Tax Asset	$ 75,065	$ 184,579