CONDENSED CONSOLIDATED STATEMENTS OF OPERATIONS AND COMPREHENSIVE INCOME - USD ($) shares in Thousands, $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2018	Sep. 30, 2017	Sep. 30, 2018	Sep. 30, 2017
CONDENSED CONSOLIDATED STATEMENTS OF OPERATIONS AND COMPREHENSIVE INCOME
Revenue	$ 233,080	$ 248,884	$ 655,087	$ 673,813
Costs and expenses
Cost of product sold (exclusive of depreciation, depletion, and accretion)	208,504	205,631	597,448	571,600
Depreciation and depletion	17,392	18,789	46,788	56,683
Accretion	1,947	1,865	5,359	5,532
(Gain) loss on derivative financial instruments	(730)	(838)	(730)	3,102
Selling, general and administrative expenses	5,101	13,059	25,393	33,861
Impairments	0	0	800	0
Other operating costs	149	121	331	429
Total costs and expenses	232,363	238,627	675,389	671,207
Operating income (loss)	717	10,257	(20,302)	2,606
Other income (expense)
Net periodic postretirement benefit income (cost), excluding service cost	20,016	1,591	23,251	4,774
Interest income	304	147	841	304
Interest expense	(8,411)	(9,573)	(28,141)	(32,351)
Other, net	67	(98)	(535)	(546)
Total other income (expense)	11,976	(7,933)	(4,584)	(27,819)
Income (loss) before income tax provision and earnings from unconsolidated affiliates	12,693	2,324	(24,886)	(25,213)
Income tax benefit (expense)	(5)	115	(302)	(36)
Income (loss) from unconsolidated affiliates, net of tax	3	138	269	771
Net income (loss)	12,691	2,577	(24,919)	(24,478)
Other comprehensive income (loss)
Postretirement medical plan amortization of prior service costs	(612)	(1,821)	(4,287)	(5,462)
Postretirement medical plan change	24,659	0	24,659	0
Postretirement medical plan termination	(19,477)	0	(19,477)	0
Income tax on postretirement medical plan	0	0	0	0
Other comprehensive income (loss)	4,570	(1,821)	895	(5,462)
Total comprehensive income (loss)	$ 17,261	$ 756	$ (24,024)	$ (29,940)
Income (loss) per common share
Basic	$ 0.17	$ 0.03	$ (0.33)	$ (0.34)
Diluted	$ 0.16	$ 0.03	$ (0.33)	$ (0.34)
Weighted-average shares outstanding - basic	75,778	75,139	75,623	72,152
Weighted-average shares outstanding - diluted	77,283	76,890	75,623	72,152