Segment Information - Reconciliation segment income to net income (loss) (Details) - USD ($) $ in Thousands	3 Months Ended		6 Months Ended
	Jun. 30, 2024	Jun. 30, 2023	Jun. 30, 2024	Jun. 30, 2023
Segment Information
Net Income (Loss)	$ 14,833	$ 77,353	$ 70,786	$ 275,461
Provision for income taxes	2,002	12,920	5,721	50,058
Interest expense, net	(1,470)	(664)	(3,254)	126
Depreciation, depletion and amortization	38,439	36,077	77,259	71,556
Accretion on asset retirement obligation	5,870	5,293	11,739	10,585
Non-service related pension and postretirement benefit costs (credits)	285	(593)	571	(1,185)
Net loss resulting from early retirement of debt				1,126
Adjusted EBITDA	59,959	130,386	162,822	407,727
EBITDA from idled or otherwise disposed operations	3,695	4,664	7,392	8,696
Selling, general and administrative expenses	22,518	22,791	48,105	48,813
Other	2,172	4,177	491	6,094
Segment Adjusted EBITDA from coal operations	$ 88,344	$ 162,018	$ 218,810	$ 471,330