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		6 Months Ended
	Jun. 30, 2024	Jun. 30, 2023	Jun. 30, 2024	Jun. 30, 2023
CWP
Defined Contribution Plan Disclosure [Line Items]
Service Cost	$ 747	$ 578	$ 1,493	$ 1,156
Interest Cost	2,066	2,072	4,132	4,143
Amortization of Actuarial Loss (Gain)	108	(262)	217	(523)
State Administrative Fees and Insurance Bond Premiums	0	0	0	0
Net Periodic Benefit Cost (Credit)	2,921	2,388	5,842	4,776
Workers' Compensation
Defined Contribution Plan Disclosure [Line Items]
Service Cost	1,464	1,399	2,928	2,798
Interest Cost	572	629	1,145	1,257
Amortization of Actuarial Loss (Gain)	(540)	(513)	(1,080)	(1,025)
State Administrative Fees and Insurance Bond Premiums	465	466	929	1,011
Net Periodic Benefit Cost (Credit)	$ 1,961	$ 1,981	$ 3,922	$ 4,041