Condensed Consolidated Income Statements - USD ($) shares in Thousands, $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2019	Sep. 30, 2018	Sep. 30, 2019	Sep. 30, 2018
Income Statement [Abstract]
Revenues	$ 619,467	$ 633,180	$ 1,744,872	$ 1,800,824
Costs, expenses and other operating
Cost of sales (exclusive of items shown separately below)	491,004	482,029	1,380,563	1,411,197
Depreciation, depletion and amortization	30,402	31,775	82,199	92,027
Accretion on asset retirement obligations	5,137	6,992	15,411	20,977
Amortization of sales contracts, net	(153)	3,241	(77)	9,540
Change in fair value of coal derivatives and coal trading activities, net	1,530	10,418	(19,851)	22,142
Selling, general and administrative expenses	24,566	22,909	73,864	73,613
Costs related to proposed joint venture with Peabody Energy	3,754	0	6,772	0
Loss on sale of Lone Mountain Processing, LLC	0	0	(4,304)	0
Preference Rights Lease Application settlement income	(39,000)	0	(39,000)	0
Other operating income, net	(4,254)	(7,070)	(9,143)	(21,320)
Costs, expenses and other operating	512,986	550,294	1,495,042	1,608,176
Income from operations	106,481	82,886	249,830	192,648
Interest expense, net
Interest expense	(4,049)	(5,179)	(12,856)	(15,624)
Interest and investment income	3,709	1,801	7,940	4,626
Interest expense, net	(340)	(3,378)	(4,916)	(10,998)
Income before nonoperating expenses	106,141	79,508	244,914	181,650
Nonoperating (expenses) income
Non-service related pension and postretirement benefit (costs) credits	975	(971)	(2,127)	(2,206)
Net loss resulting from early retirement of debt and debt restructuring	0	0	0	(485)
Reorganization items, net	0	(560)	71	(1,601)
Nonoperating expenses	975	(1,531)	(2,056)	(4,292)
Income before income taxes	107,116	77,977	242,858	177,358
Provision for (benefit from) income taxes	347	(45,215)	508	(49,125)
Net income	$ 106,769	$ 123,192	$ 242,350	$ 226,483
Net income per common share
Basic earnings per common share (in dollars per share)	$ 6.79	$ 6.40	$ 14.61	$ 11.27
Diluted earnings per common share (in dollars per share)	$ 6.34	$ 6.10	$ 13.66	$ 10.76
Weighted average shares outstanding
Basic weighted average shares outstanding (in shares)	15,736	19,250	16,591	20,102
Diluted weighted average shares outstanding (in shares)	16,852	20,208	17,744	21,040
Dividends declared per common share (in dollars per share)	$ 0.45	$ 0.40	$ 1.35	$ 1.20