Employee Benefit Plans- Net Periodic Benefit Cost (Details) - USD ($) $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2023	Sep. 30, 2022	Sep. 30, 2023	Sep. 30, 2022
Pension
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Interest cost	$ 5,993	$ 3,995	$ 17,979	$ 11,985
Expected return on plan assets	(5,499)	(7,183)	(16,497)	(21,549)
Amortization of net actuarial gain (loss)	183	528	548	1,583
Net periodic benefit cost (credit)	677	(2,660)	2,030	(7,981)
Black Lung
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Service cost	512	661	1,538	1,982
Interest cost	1,165	681	3,495	2,041
Expected return on plan assets	(12)	(13)	(38)	(39)
Amortization of net actuarial gain (loss)	(708)	314	(2,124)	943
Net periodic benefit cost (credit)	$ 957	$ 1,643	$ 2,871	$ 4,927