Income Taxes - Schedule of Deferred Tax Assets and Liabilities (Details) - USD ($) $ in Thousands	Dec. 31, 2022	Dec. 31, 2021
Deferred income tax assets:
Net operating loss and credit carryforwards	$ 98,533	$ 226,438
Inventory	1,560	494
Asset retirement obligations	14,466	14,930
Black lung obligations	6,391	7,834
Accrued expenses	5,955	6,365
Other	864	268
Total deferred income tax assets	127,769	256,329
Less: valuation allowance for deferred income tax assets	(41,433)	(45,952)
Net deferred income tax assets	86,336	210,377
Deferred income tax liabilities:
Inventory	0	0
Prepaid expenses	(8,308)	(8,444)
Property, plant and equipment	(92,748)	(74,339)
Other	(1,086)	(2,318)
Total deferred income tax liabilities	(102,142)	(85,101)
Net deferred income tax (liability) asset	$ (15,806)
Net deferred income tax (liability) asset		$ 125,276