Components of Coal Workers' Pneumoconiosis (CWP) and Workers' Compensation Net Periodic Benefit Costs - Components of Net Period Benefit Costs (Details) - USD ($) $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2023	Sep. 30, 2022	Sep. 30, 2023	Sep. 30, 2022
CWP
Defined Contribution Plan Disclosure [Line Items]
Service Cost	$ 578	$ 727	$ 1,734	$ 2,179
Interest Cost	2,071	1,265	6,214	3,795
Amortization of Actuarial Loss	(261)	1,059	(784)	3,178
State Administrative Fees and Insurance Bond Premiums	0	0	0	0
Net Periodic Benefit (Credit) Cost	2,388	3,051	7,164	9,152
Workers' Compensation
Defined Contribution Plan Disclosure [Line Items]
Service Cost	1,400	1,230	4,198	3,690
Interest Cost	628	343	1,885	1,027
Amortization of Actuarial Loss	(512)	(106)	(1,537)	(316)
State Administrative Fees and Insurance Bond Premiums	471	449	1,482	1,334
Net Periodic Benefit (Credit) Cost	$ 1,987	$ 1,916	$ 6,028	$ 5,735