CONDENSED CONSOLIDATED STATEMENTS OF OPERATIONS AND COMPREHENSIVE INCOME - USD ($) shares in Thousands, $ in Thousands	3 Months Ended		6 Months Ended
	Jun. 30, 2017	Jun. 30, 2016	Jun. 30, 2017	Jun. 30, 2016
CONDENSED CONSOLIDATED STATEMENTS OF OPERATIONS AND COMPREHENSIVE INCOME
Revenue	$ 229,201	$ 174,188	$ 424,930	$ 355,437
Costs and expenses
Cost of product sold (exclusive of depreciation, depletion, and accretion)	194,370	140,616	363,309	305,650
Depreciation and depletion	19,249	(19,510)	37,894	(408)
Accretion	1,846	1,994	3,667	4,576
(Gain) loss on derivative financial instruments	1,595	(8,286)	3,939	(6,325)
Selling, general and administrative expenses	9,565	13,251	20,279	27,026
Impairments	0	34	0	4,187
Other operating costs	92	169	310	456
Total costs and expenses	226,717	128,268	429,398	335,162
Operating income (loss)	2,484	45,920	(4,468)	20,275
Other income (expense)
Interest income	118	33	157	70
Interest expense	(9,866)	(11,286)	(22,778)	(22,338)
Other, net	(138)	(206)	(448)	(595)
Total other income (expense)	(9,886)	(11,459)	(23,069)	(22,863)
Income (loss) before income tax provision and earnings from unconsolidated affiliates	(7,402)	34,461	(27,537)	(2,588)
Income tax benefit (expense)	149	1,158	(151)	2,580
Income (loss) from unconsolidated affiliates, net of tax	305	(330)	632	(1,078)
Net income (loss)	(6,948)	35,289	(27,056)	(1,086)
Other comprehensive income (loss)
Postretirement medical plan amortization of prior service costs	(1,821)	(1,871)	(3,642)	(1,510)
Postretirement medical plan change		42,851	0	42,851
Income tax on postretirement medical and pension changes	0	(974)	0	(1,944)
Other comprehensive income (loss)	(1,821)	40,006	(3,642)	39,397
Total comprehensive income (loss)	$ (8,769)	$ 75,295	$ (30,698)	$ 38,311
Income (loss) per common share
Basic	$ (0.09)	$ 0.58	$ (0.38)	$ (0.02)
Diluted	$ (0.09)	$ 0.57	$ (0.38)	$ (0.02)
Weighted-average shares outstanding - basic	75,086	61,296	70,634	61,244
Weighted-average shares outstanding - diluted	75,086	61,971	70,634	61,244