Other Accrued Liabilities (Schedule of Other Accrued Liabilities) (Details) - USD ($) $ in Thousands	Dec. 31, 2017	Dec. 31, 2016
Other Liabilities Disclosure [Abstract]
Subsidence liability	$ 88,027	$ 104,437
Longwall equipment buyout	22,631	0
Accrued payroll and benefits	14,689	17,326
Accrued interest	10,039	2,239
Equipment lease rental	9,865	15,286
Litigation	8,197	12,532
Accrued other taxes	7,510	12,732
Deferred revenue	6,807	10,520
Short-term incentive compensation	4,729	6,073
Other	23,900	19,747
Current portion of long-term liabilities:
Postretirement benefits other than pensions	37,464	40,611
Asset retirement obligations	30,480	26,259
Workers' compensation	13,317	13,596
Pneumoconiosis benefits	12,972	10,763
Total Other Accrued Liabilities	$ 290,627	$ 292,121