Condensed Consolidated Income Statements - USD ($) shares in Thousands, $ in Thousands	3 Months Ended		6 Months Ended
	Jun. 30, 2018	Jun. 30, 2017	Jun. 30, 2018	Jun. 30, 2017
Income Statement [Abstract]
Revenues	$ 592,349	$ 549,866	$ 1,167,644	$ 1,150,841
Costs, expenses and other operating
Cost of sales (exclusive of items shown separately below)	474,388	434,465	929,168	894,915
Depreciation, depletion and amortization	30,549	30,701	60,252	62,622
Accretion on asset retirement obligations	6,993	7,623	13,985	15,246
Amortization of sales contracts, net	3,248	14,352	6,299	29,042
Change in fair value of coal derivatives and coal trading activities, net	15,138	863	11,724	1,717
Selling, general and administrative expenses	24,756	22,456	50,704	43,218
Other operating income, net	(7,318)	(3,518)	(14,250)	(5,828)
Costs, expenses and other operating	547,754	506,942	1,057,882	1,040,932
Income from operations	44,595	42,924	109,762	109,909
Interest expense, net
Interest expense	(5,050)	(6,003)	(10,445)	(15,428)
Interest and investment income	1,552	842	2,825	1,369
Interest expense, net	(3,498)	(5,161)	(7,620)	(14,059)
Income before nonoperating expenses	41,097	37,763	102,142	95,850
Nonoperating expenses
Non-service related pension and postretirement benefit (costs) credits	68	(232)	(1,235)	(953)
Net loss resulting from early retirement of debt and debt restructuring	(485)	(31)	(485)	(2,061)
Reorganization items, net	(740)	(21)	(1,041)	(2,849)
Nonoperating expenses	(1,157)	(284)	(2,761)	(5,863)
Income before income taxes	39,940	37,479	99,381	89,987
Provision for (benefit from) income taxes	(3,366)	319	(3,910)	1,159
Net income	$ 43,306	$ 37,160	$ 103,291	$ 88,828
Net income per common share
Basic earnings per common share (in dollars per share)	$ 2.15	$ 1.51	$ 5.03	$ 3.58
Diluted earnings per common share (in dollars per share)	$ 2.06	$ 1.48	$ 4.81	$ 3.52
Weighted average shares outstanding
Basic weighted average shares outstanding (in shares)	20,156	24,659	20,529	24,834
Diluted weighted average shares outstanding (in shares)	21,036	25,082	21,456	25,245
Dividends declared per common share (in dollars per share)	$ 0.4	$ 0.35	$ 0.8	$ 0.35