Condensed Consolidated Income Statements - USD ($) shares in Thousands, $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2018	Sep. 30, 2017	Sep. 30, 2018	Sep. 30, 2017
Income Statement [Abstract]
Revenues	$ 633,180	$ 613,538	$ 1,800,824	$ 1,764,379
Costs, expenses and other operating
Cost of sales (exclusive of items shown separately below)	482,029	494,379	1,411,197	1,389,294
Depreciation, depletion and amortization	31,775	31,914	92,027	94,536
Accretion on asset retirement obligations	6,992	7,580	20,977	22,826
Amortization of sales contracts, net	3,241	13,861	9,540	42,903
Change in fair value of coal derivatives and coal trading activities, net	10,418	1,028	22,142	2,745
Selling, general and administrative expenses	22,909	21,290	73,613	64,508
Gain on sale of Lone Mountain Processing, Inc.	0	(21,574)	0	(21,574)
Other operating income, net	(7,070)	(8,250)	(21,320)	(14,078)
Costs, expenses and other operating	550,294	540,228	1,608,176	1,581,160
Income from operations	82,886	73,310	192,648	183,219
Interest expense, net
Interest expense	(5,179)	(5,972)	(15,624)	(21,400)
Interest and investment income	1,801	720	4,626	2,089
Interest expense, net	(3,378)	(5,252)	(10,998)	(19,311)
Income before nonoperating expenses	79,508	68,058	181,650	163,908
Nonoperating expenses
Non-service related pension and postretirement benefit costs	(971)	(821)	(2,206)	(1,774)
Net loss resulting from early retirement of debt and debt restructuring	0	(486)	(485)	(2,547)
Reorganization items, net	(560)	(43)	(1,601)	(2,892)
Nonoperating expenses	(1,531)	(1,350)	(4,292)	(7,213)
Income before income taxes	77,977	66,708	177,358	156,695
Benefit from income taxes	(45,215)	(1,643)	(49,125)	(484)
Net income	$ 123,192	$ 68,351	$ 226,483	$ 157,179
Net income per common share
Basic earnings per common share (in dollars per share)	$ 6.40	$ 2.90	$ 11.27	$ 6.44
Diluted earnings per common share (in dollars per share)	$ 6.10	$ 2.83	$ 10.76	$ 6.32
Weighted average shares outstanding
Basic weighted average shares outstanding (in shares)	19,250	23,580	20,102	24,416
Diluted weighted average shares outstanding (in shares)	20,208	24,135	21,040	24,875
Dividends declared per common share (in dollars per share)	$ 0.4	$ 0.35	$ 1.2	$ 0.7