Condensed Consolidated Income Statements - USD ($) shares in Thousands, $ in Thousands	12 Months Ended
	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
Income Statement [Abstract]
Revenues	$ 1,467,592	$ 2,294,352	$ 2,451,787
Costs, expenses and other operating
Cost of sales (exclusive of items shown separately below)	1,378,479	1,873,017	1,925,202
Depreciation, depletion and amortization	121,552	111,621	130,670
Accretion on asset retirement obligations	19,887	20,548	27,970
Change in fair value of coal derivatives and coal trading activities, net	5,219	(18,601)	9,118
Selling, general and administrative expenses	82,397	95,781	100,300
Costs related to proposed joint venture with Peabody Energy	16,087	13,816
Asset impairment and restructuring	221,380
Gain on property insurance recovery related to Mountain Laurel longwall	(23,518)	0
(Gain) loss on divestitures	(1,505)	13,312
Preference Rights Lease Application settlement income	0	(39,000)
Other operating income, net	(22,246)	(19,012)	(20,611)
Costs, expenses and other operating	1,797,732	2,051,482	2,172,649
Income (loss) from operations	(330,140)	242,870	279,138
Interest expense, net
Interest expense	(14,432)	(16,485)	(20,471)
Interest and investment income	3,808	9,691	6,782
Interest expense, net	(10,624)	(6,794)	(13,689)
Income (loss) before nonoperating expenses	(340,764)	236,076	265,449
Nonoperating (expenses) income
Non-service related pension and postretirement benefit (costs) credits	(3,884)	(2,053)	(3,202)
Net loss resulting from early retirement of debt and debt restructuring			(485)
Reorganization items, net	26	24	(1,661)
Nonoperating (expenses) income	(3,858)	(2,029)	(5,348)
Income (loss) before income taxes	(344,622)	234,047	260,101
Provision for (benefit from) income taxes	(7)	248	(52,476)
Net income (loss)	$ (344,615)	$ 233,799	$ 312,577
Net income (loss) per common share
Basic earnings (loss) per common share (in dollars per share)	$ (22.74)	$ 14.42	$ 15.90
Diluted earnings (loss) per common share (in dollars per share)	$ (22.74)	$ 13.52	$ 15.15
Weighted average shares outstanding
Basic weighted average shares outstanding (in shares)	15,153	16,218	19,663
Diluted weighted average shares outstanding (in shares)	15,153	17,298	20,629
Dividends declared per common share (in dollars per share)	$ 0.50	$ 1.80