Condensed Consolidated Income Statements - USD ($) shares in Thousands, $ in Thousands	3 Months Ended
	Mar. 31, 2019	Mar. 31, 2018
Income Statement [Abstract]
Revenues	$ 555,183	$ 575,295
Costs, expenses and other operating
Cost of sales (exclusive of items shown separately below)	438,471	454,780
Depreciation, depletion and amortization	25,273	29,703
Accretion on asset retirement obligations	5,137	6,992
Amortization of sales contracts, net	65	3,051
Change in fair value of coal derivatives and coal trading activities, net	(12,981)	(3,414)
Selling, general and administrative expenses	24,089	25,948
Other operating income, net	(1,650)	(6,932)
Costs, expenses and other operating	478,404	510,128
Income from operations	76,779	65,167
Interest expense, net
Interest expense	(4,432)	(5,395)
Interest and investment income	2,143	1,273
Interest expense, net	(2,289)	(4,122)
Income before nonoperating expenses	74,490	61,045
Nonoperating expenses
Non-service related pension and postretirement benefit costs	(1,766)	(1,303)
Reorganization items, net	87	(301)
Nonoperating expenses	(1,679)	(1,604)
Income before income taxes	72,811	59,441
Provision for (benefit from) income taxes	70	(544)
Net income	$ 72,741	$ 59,985
Net income per common share
Basic earnings per common share (in dollars per share)	$ 4.16	$ 2.87
Diluted earnings per common share (in dollars per share)	$ 3.91	$ 2.74
Weighted average shares outstanding
Basic weighted average shares outstanding (in shares)	17,494	20,901
Diluted weighted average shares outstanding (in shares)	18,599	21,875
Dividends declared per common share (in dollars per share)	$ 0.45	$ 0.40