Segment Information - Reconciliation segment income to net income (loss) (Details) - USD ($) $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2023	Sep. 30, 2022	Sep. 30, 2023	Sep. 30, 2022
Segment Information
Net Income (Loss)	$ 73,691	$ 181,007	$ 349,152	$ 860,443
Provision for income taxes	16,781	474	66,839	1,424
Interest expense, net	(1,683)	1,836	(1,557)	13,469
Depreciation, depletion and amortization	36,717	33,958	108,273	98,948
Accretion on asset retirement obligation	5,292	4,430	15,877	13,290
Non-service related pension and postretirement benefit (credits) costs	(4,507)	857	(5,692)	2,189
Net loss resulting from early retirement of debt		394	1,126	14,143
Adjusted EBITDA	126,291	222,956	534,018	1,003,906
EBITDA from idled or otherwise disposed operations	30	3,624	8,726	9,972
Selling, general and administrative expenses	24,279	26,107	73,092	79,271
Other	1,095	(690)	7,189	8,114
Segment Adjusted EBITDA from coal operations	$ 151,695	$ 251,997	$ 623,025	$ 1,101,263