Condensed Consolidated Statements of Operations - USD ($) shares in Thousands, $ in Thousands	3 Months Ended
	Mar. 31, 2018	Mar. 31, 2017
Income Statement [Abstract]
Revenues	$ 575,295	$ 600,975
Costs, expenses and other operating
Cost of sales (exclusive of items shown separately below)	454,780	460,450
Depreciation, depletion and amortization	29,703	31,921
Accretion on asset retirement obligations	6,992	7,623
Amortization of sales contracts, net	3,051	14,690
Change in fair value of coal derivatives and coal trading activities, net	(3,414)	854
Selling, general and administrative expenses	25,948	20,762
Other operating income, net	(6,932)	(2,310)
Total operating expenses	510,128	533,990
Income from operations	65,167	66,985
Interest expense, net
Interest expense	(5,395)	(9,425)
Interest and investment income	1,273	527
Interest expense, net	(4,122)	(8,898)
Income before nonoperating expenses	61,045	58,087
Nonoperating expenses
Non-service related pension and postretirement benefit costs	(1,303)	(721)
Net loss resulting from early retirement of debt and debt restructuring	0	(2,030)
Reorganization items, net	(301)	(2,828)
Nonoperating income (expense)	(1,604)	(5,579)
Income before income taxes	59,441	52,508
Provision for (benefit from) income taxes	(544)	840
Net income	$ 59,985	$ 51,668
Net income per common share
Basic earnings (loss) per common share (in dollars per share)	$ 2.87	$ 2.07
Diluted earnings (loss) per common share (in dollars per share)	$ 2.74	$ 2.03
Weighted average shares outstanding
Basic weighted average shares outstanding (shares)	20,901	25,008
Diluted weighted average shares outstanding (shares)	21,875	25,408
Dividends declared per common share (in dollars per share)	$ 0.4	$ 0