Other Accrued Liabilities (Schedule of Other Accrued Liabilities) (Details) - USD ($) $ in Thousands	Dec. 31, 2018	Dec. 31, 2017
Other Liabilities Disclosure [Abstract]
Subsidence Liability	$ 83,532	$ 88,027
Accrued Payroll and Benefits	12,978	14,689
Litigation	8,235	8,197
Accrued Interest	6,850	10,039
Short-Term Incentive Compensation	6,024	4,729
Accrued Other Taxes	5,050	7,510
Deferred Revenue	151	6,807
Equipment Lease Rental	0	9,865
Longwall Equipment Buyout	0	22,631
Other	15,437	23,900
Current Portion of Long-Term Liabilities:
Postretirement Benefits Other than Pensions	32,345	37,464
Asset Retirement Obligations	31,017	30,480
Workers' Compensation	12,628	13,317
Pneumoconiosis Benefits	12,187	12,972
Total Other Accrued Liabilities	$ 226,434	$ 290,627