Condensed Consolidated Income Statements - USD ($) shares in Thousands, $ in Thousands	3 Months Ended
	Mar. 31, 2020	Mar. 31, 2019
Income Statement [Abstract]
Revenues	$ 405,232	$ 555,183
Costs, expenses and other operating
Cost of sales (exclusive of items shown separately below)	374,999	438,471
Depreciation, depletion and amortization	31,308	25,273
Accretion on asset retirement obligations	5,006	5,137
Amortization of sales contracts, net		65
Change in fair value of coal derivatives and coal trading activities, net	743	(12,981)
Selling, general and administrative expenses	22,745	24,089
Costs related to proposed joint venture with Peabody Energy	3,664
Severance costs related to voluntary separation plan	5,828
Property insurance recovery related to Mountain Laurel longwall	(9,000)
Other operating income, net	(6,170)	(1,650)
Costs, expenses and other operating	429,123	478,404
Income (loss) from operations	(23,891)	76,779
Interest expense, net
Interest expense	(3,388)	(4,432)
Interest and investment income	1,259	2,143
Interest expense, net	(2,129)	(2,289)
Income (loss) before nonoperating expenses	(26,020)	74,490
Nonoperating (expenses) income
Non-service related pension and postretirement benefit costs	(1,096)	(1,766)
Reorganization items, net	26	87
Nonoperating (expenses) income	(1,070)	(1,679)
Income (loss) before income taxes	(27,090)	72,811
Provision for (benefit from) income taxes	(1,791)	70
Net income (loss)	$ (25,299)	$ 72,741
Net income (loss) per common share
Basic earnings per common share (in dollars per share)	$ (1.67)	$ 4.16
Diluted earnings per common share (in dollars per share)	$ (1.67)	$ 3.91
Weighted average shares outstanding
Basic weighted average shares outstanding (in shares)	15,139	17,494
Diluted weighted average shares outstanding (in shares)	15,139	18,599
Dividends declared per common share (in dollars per share)	$ 0.50	$ 0.45