Other Accrued Liabilities (Schedule of Other Accrued Liabilities) (Details) - USD ($) $ in Thousands	Dec. 31, 2019	Dec. 31, 2018
Other Liabilities Disclosure [Abstract]
Subsidence Liability	$ 90,645	$ 83,532
Accrued Payroll and Benefits	21,102	12,978
Accrued Interest	6,281	6,850
Accrued Other Taxes	4,753	5,050
Short-Term Incentive Compensation	3,997	6,024
Litigation	2,565	8,235
Other	9,719	15,588
Current Portion of Long-Term Liabilities:
Postretirement Benefits Other than Pensions	31,833	32,345
Asset Retirement Obligations	21,741	31,017
Operating Lease Liability	19,479
Pneumoconiosis Benefits	12,331	12,187
Workers' Compensation	11,323	12,628
Total Other Accrued Liabilities	$ 235,769	$ 226,434