Other Accrued Liabilities (Details) - USD ($) $ in Thousands	Dec. 31, 2016	Dec. 31, 2015
Other Liabilities Disclosure [Abstract]
Derivative Instruments and Hedges, Liabilities	$ 231,573	$ 12,206
Subsidence Liability	104,437	86,860
Accrued Royalties	44,657	38,707
Interest Payable, Current	37,366	38,406
Accrual for Taxes Other than Income Taxes, Current	20,679	10,927
Accrued Bonuses, Current	19,497	6,802
Deferred Revenue	18,211	11,557
Employee-related Liabilities	24,649	24,952
Lease Incentive, Payable, Current	15,286	15,286
Accrued Liabilities and Other Liabilities	59,025	60,052
Other Postretirement Benefits Payable	42,001	46,105
Accrued Reclamation Costs, Current	0	515
Pneumoconiosis Liability, Current	10,763	9,382
Defined Benefit Pension Plan Liabilities, Current	2,904	2,772
Long Term Disability, Current	3,865	4,248
Mine closing liability, current	14,276	22,599
Gas well closing liability, current	17,285	15,648
Workers' Compensation Liability, Current	13,874	14,803
Other Accrued Liabilities	$ 680,348	$ 421,827