CONDENSED CONSOLIDATED STATEMENTS OF OPERATIONS AND COMPREHENSIVE INCOME - USD ($) shares in Thousands, $ in Thousands	3 Months Ended								12 Months Ended
	Dec. 31, 2015	Sep. 30, 2015	Jun. 30, 2015	Mar. 31, 2015	Dec. 31, 2014	Sep. 30, 2014	Jun. 30, 2014	Mar. 31, 2014	Dec. 31, 2015	Dec. 31, 2014	Dec. 31, 2013
CONDENSED CONSOLIDATED STATEMENTS OF OPERATIONS AND COMPREHENSIVE INCOME
Revenue	$ 260,737	$ 301,673	$ 244,148	$ 317,553	$ 341,791	$ 342,337	$ 320,850	$ 319,066	$ 1,124,111	$ 1,324,044	$ 1,396,097
Costs and expenses
Cost of product sold (exclusive of depreciation, depletion, and accretion, shown separately)									950,580	1,094,257	1,136,318
Depreciation and depletion									66,064	112,022	100,523
Amortization of port access rights (Note 10)									3,710	0	0
Accretion (Note 17)									12,555	15,136	15,342
(Gain) loss on derivative financial instruments (Note 7)									30,635	(7,805)	(25,611)
Selling, general and administrative expenses									48,925	50,201	53,066
Impairments (Note 9)									91,541	0	0
Other operating costs									1,492	2,693	4,077
Total costs and expenses									1,205,502	1,266,504	1,283,715
Gain on sale of Decker Mine interest (Note 4)						(74,300)			0	(74,262)	0
Operating income	(56,022)	17,458	(50,812)	7,984	22,407	85,844	7,428	16,124	(81,391)	131,802	112,382
Other income (expense)
Interest income									170	259	440
Interest expense (Note 16)									(47,561)	(77,160)	(41,665)
Tax agreement benefit (expense) (Note 12)									0	58,595	(10,515)
Other, net									62	(202)	2,423
Total other income (expense)									(47,329)	(18,508)	(49,317)
Income (loss) before income tax provision and earnings from unconsolidated affiliates									(128,720)	113,294	63,065
Income tax benefit (expense) (Note 18)									(77,380)	(34,913)	(11,629)
Income (loss) from unconsolidated affiliates, net of tax									1,200	579	535
Net income (loss)	$ (156,196)	$ 8,873	$ (52,897)	$ (4,680)	$ 5,665	$ 91,069	$ (2,148)	$ (15,626)	(204,900)	78,960	51,971
Other comprehensive income (loss)
Postretirement medical plan amortization of prior service costs (Note 19)									1,252	989	1,775
Postretirement medical plan adjustment (Note 19)									(3,874)	(5,564)	10,824
Decker pension adjustments									0	0	3,199
Write-off of prior service costs related to Decker pension plan (Note 22)									0	3,183	0
Income tax on postretirement medical and pension adjustments									970	372	(5,616)
Other comprehensive income (loss)									(1,652)	(1,020)	10,182
Total comprehensive income (loss)									$ (206,552)	$ 77,940	$ 62,153
Income (loss) per common share (Note 23)
Basic	$ (2.55)	$ 0.15	$ (0.87)	$ (0.08)	$ 0.09	$ 1.5	$ (0.04)	$ (0.26)	$ (3.36)	$ 1.3	$ 0.86
Diluted	$ (2.55)	$ 0.14	$ (0.87)	$ (0.08)	$ 0.09	$ 1.49	$ (0.04)	$ (0.26)	$ (3.36)	$ 1.29	$ 0.85
Weighted-average shares outstanding - basic									61,053	60,826	60,652
Weighted-average shares outstanding - diluted									61,053	61,295	61,161