Coal Workers' Pneumoconiosis and Workers' Compensation - Schedule of Net Benefit Costs (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2023	Dec. 31, 2022	Dec. 31, 2021
CWP Benefits
Defined Benefit Plan Disclosure [Line Items]
Service cost	$ 2,313	$ 2,905	$ 4,460
Interest cost	8,285	5,060	4,710
Recognized net actuarial loss	(1,045)	4,238	8,364
State administrative fees and insurance bond premiums	0	0	0
Net periodic benefit (credit) cost	9,553	12,203	17,534
Workers Compensation
Defined Benefit Plan Disclosure [Line Items]
Service cost	5,597	4,920	4,236
Interest cost	2,514	1,369	1,127
Recognized net actuarial loss	(2,049)	(420)	(179)
State administrative fees and insurance bond premiums	1,953	1,817	1,778
Net periodic benefit (credit) cost	$ 8,015	$ 7,686	$ 6,962