CONDENSED CONSOLIDATED STATEMENTS OF OPERATIONS AND COMPREHENSIVE INCOME - USD ($) shares in Thousands, $ in Thousands		3 Months Ended								12 Months Ended
	Oct. 17, 2016	Dec. 31, 2017	Sep. 30, 2017	Jun. 30, 2017	Mar. 31, 2017	Dec. 31, 2016	Sep. 30, 2016	Jun. 30, 2016	Mar. 31, 2016	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
CONDENSED CONSOLIDATED STATEMENTS OF OPERATIONS AND COMPREHENSIVE INCOME
Revenue		$ 213,893	$ 248,884	$ 229,201	$ 195,728	$ 227,928	$ 217,073	$ 174,188	$ 181,249	$ 887,706	$ 800,438	$ 1,124,111
Costs and expenses
Cost of product sold (exclusive of depreciation, depletion, and accretion, shown separately)										752,715	646,404	950,580
Depreciation and depletion										72,270	27,218	66,064
Amortization of port access rights										0	0	3,710
Accretion (Note 14)										7,072	6,645	12,555
(Gain) loss on derivative financial instruments (Note 6)										2,672	(8,180)	30,635
Selling, general and administrative expenses										47,482	50,868	48,925
Impairments (Note 8)									4,200	0	4,609	91,541
Debt restructuring costs	$ 4,700					200	4,500			23	4,665	0
Other operating costs										532	941	1,492
Total costs and expenses										882,766	733,170	1,205,502
Operating income (loss)		(2,441)	11,848	2,484	(6,951)	36,131	10,861	45,920	(25,645)	4,940	67,268	(81,391)
Other income (expense)
Interest income										485	138	170
Interest expense (Note 13)										(41,362)	(47,434)	(47,561)
Other, net										(885)	(1,001)	62
Total other income (expense)										(41,762)	(48,297)	(47,329)
Income (loss) before income tax provision and earnings from unconsolidated affiliates										(36,822)	18,971	(128,720)
Income tax benefit (expense) (Note 15)										29,470	2,213	(77,380)
Income (loss) from unconsolidated affiliates, net of tax (Note 10)										713	657	1,200
Net income (loss)		$ 17,839	$ 2,577	$ (6,948)	$ (20,108)	$ 24,511	$ (1,584)	$ 35,289	$ (36,375)	(6,639)	21,841	(204,900)
Other comprehensive income (loss)
Postretirement medical plan amortization of prior service costs (Note 16)										(7,283)	(5,253)	1,252
Postretirement medical plan adjustments (Note 16)										(794)	(1,792)	(3,874)
Postretirement medical plan change (Note 16)										0	42,851	0
Income tax on postretirement medical and pension changes										0	(971)	970
Other comprehensive income (loss)										(8,077)	34,835	(1,652)
Total comprehensive income (loss)										$ (14,716)	$ 56,676	$ (206,552)
Income (loss) per common share
Basic		$ 0.24	$ 0.03	$ (0.09)	$ (0.3)	$ 0.4	$ (0.03)	$ 0.58	$ (0.59)	$ (0.09)	$ 0.36	$ (3.36)
Diluted		$ 0.23	$ 0.03	$ (0.09)	$ (0.3)	$ 0.39	$ (0.03)	$ 0.57	$ (0.59)	$ (0.09)	$ 0.35	$ (3.36)
Weighted-average shares outstanding - basic										72,907	61,328	61,053
Weighted-average shares outstanding - diluted										72,907	62,290	61,053