Segment Information - Reconciliation segment income to net income (loss) (Details) - USD ($) $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2024	Sep. 30, 2023	Sep. 30, 2024	Sep. 30, 2023
Segment Information
Net income (loss)	$ (6,221)	$ 73,691	$ 64,565	$ 349,152
(Benefit from) provision for income taxes	(6,608)	16,781	(887)	66,839
Interest expense, net	(1,753)	(1,683)	(5,007)	(1,557)
Depreciation, depletion and amortization	40,890	36,717	118,149	108,273
Accretion on asset retirement obligation	5,869	5,292	17,608	15,877
Merger related costs	7,002		7,002
Severance costs related to voluntary separation plan	6,649		6,649
Non-service related pension and postretirement benefit credits	(1,667)	(4,507)	(1,096)	(5,692)
Net loss resulting from early retirement of debt				1,126
Adjusted EBITDA	44,161	126,291	206,983	534,018
EBITDA from idled or otherwise disposed operations	4,101	30	11,493	8,726
Selling, general and administrative expenses	20,603	24,279	68,708	73,092
Other	(1,851)	1,095	(1,360)	7,189
Segment Adjusted EBITDA from coal operations	$ 67,014	$ 151,695	$ 285,824	$ 623,025