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
	Mar. 31, 2024	Mar. 31, 2023
CWP
Defined Contribution Plan Disclosure [Line Items]
Service Cost	$ 746	$ 578
Interest Cost	2,066	2,071
Amortization of Actuarial Loss (Gain)	109	(261)
State Administrative Fees and Insurance Bond Premiums	0	0
Net Periodic Benefit Cost (Credit)	2,921	2,388
Workers' Compensation
Defined Contribution Plan Disclosure [Line Items]
Service Cost	1,464	1,399
Interest Cost	573	628
Amortization of Actuarial Loss (Gain)	(540)	(512)
State Administrative Fees and Insurance Bond Premiums	464	545
Net Periodic Benefit Cost (Credit)	$ 1,961	$ 2,060