Condensed Consolidated Income Statements - USD ($) shares in Thousands, $ in Thousands	3 Months Ended		6 Months Ended
	Jun. 30, 2021	Jun. 30, 2020	Jun. 30, 2021	Jun. 30, 2020
Income Statement [Abstract]
Revenues	$ 450,389	$ 319,521	$ 807,932	$ 724,753
Costs, expenses and other operating
Cost of sales (exclusive of items shown separately below)	355,329	316,348	665,235	691,347
Depreciation, depletion and amortization	27,884	30,167	53,681	61,475
Accretion on asset retirement obligations	5,437	4,986	10,874	9,992
Change in fair value of coal derivatives and coal trading activities, net	8,762	(129)	9,290	614
Selling, general and administrative expenses	24,119	19,738	45,599	42,483
Costs related to proposed joint venture with Peabody Energy		7,851		11,515
Asset impairment and restructuring		7,437		13,265
Gain on property insurance recovery related to Mountain Laurel longwall		(14,518)		(23,518)
Gain on divestitures		(1,369)		(1,369)
Other operating income, net	(4,347)	(5,704)	(9,615)	(11,874)
Costs, expenses and other operating	417,184	364,807	775,064	793,930
Income (loss) from operations	33,205	(45,286)	32,868	(69,177)
Interest expense, net
Interest expense	(2,941)	(3,523)	(7,069)	(6,911)
Interest and investment income	147	1,793	474	3,052
Interest expense, net	(2,794)	(1,730)	(6,595)	(3,859)
Income (loss) before nonoperating expenses	30,411	(47,016)	26,273	(73,036)
Nonoperating (expenses) income
Non-service related pension and postretirement benefit costs	(539)	(1,102)	(2,066)	(2,198)
Reorganization items, net				26
Nonoperating (expenses) income	(539)	(1,102)	(2,066)	(2,172)
Income (loss) before income taxes	29,872	(48,118)	24,207	(75,208)
Provision for (benefit from) income taxes	2,006	1,206	2,383	(585)
Net income (loss)	$ 27,866	$ (49,324)	$ 21,824	$ (74,623)
Net income (loss) per common share
Basic earnings (loss) per common share (in dollars per share)	$ 1.82	$ (3.26)	$ 1.43	$ (4.93)
Diluted earnings (loss) per common share (in dollars per share)	$ 1.66	$ (3.26)	$ 1.31	$ (4.93)
Weighted average shares outstanding
Basic weighted average shares outstanding (in shares)	15,294	15,145	15,289	15,142
Diluted weighted average shares outstanding (in shares)	16,756	15,145	16,598	15,142
Dividends declared per common share (in dollars per share)				$ 0.50