Employee Benefit Plans - Changes in Plan Assets and Benefit Obligations Recognized in Other Comprehensive (Loss) Income (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2023	Dec. 31, 2022	Dec. 31, 2021
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Actuarial loss (gain)	$ 34,205	$ (56,485)	$ (47,461)
Amortization of net actuarial gain (loss)	2,324	(3,555)	(6,021)
Pension Plan
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Actuarial loss (gain)	14,106	(32,912)	(37,004)
Amortization of net actuarial gain (loss)	(730)	(2,111)	(3,217)
Settlement	0	(244)	(412)
Total recognized in other comprehensive income (loss)	13,376	(35,267)	(40,633)
Black Lung
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Actuarial loss (gain)	19,995	(20,881)	(9,649)
Amortization of net actuarial gain (loss)	2,833	(1,257)	(2,453)
Total recognized in other comprehensive income (loss)	$ 22,828	$ (22,138)	$ (12,102)