QUARTERLY FINANCIAL DATA (UNAUDITED) Quarterly Financial Data - Income Statement (Details) - USD ($) $ / shares in Units, shares in Thousands, $ in Thousands	3 Months Ended									6 Months Ended		9 Months Ended		12 Months Ended
	Dec. 31, 2016	Sep. 30, 2016	Jun. 30, 2016	Mar. 31, 2016	Dec. 31, 2015	[1]	Sep. 30, 2015	Jun. 30, 2015	Mar. 31, 2015	Jun. 30, 2016	Jun. 30, 2015	Sep. 30, 2016	Sep. 30, 2015	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014
Revenues	$ 392,737	$ 371,772	$ 357,597	$ 355,854	$ 341,666		$ 352,000	$ 351,737	$ 374,115	$ 713,451	$ 725,855	$ 1,085,223	$ 1,077,854	$ 1,477,960	$ 1,419,518	$ 1,131,000
Cost of sales		285,428	298,181	281,125			302,269	292,704	309,712	579,307	602,414	864,735	904,682	1,156,687	1,175,849	930,268
Depreciation, depletion and amortization		40,860	35,223	37,015			37,240	36,332	39,908	72,237	76,240	113,097	113,480	185,267	140,328	109,361
Selling and administrative		25,655	27,613	27,399			25,057	23,924	22,380	55,012	46,308	80,667	71,365	108,560	95,554	82,304
Heritage health benefit expenses		3,265	3,222	3,015			2,801	2,162	3,059	6,237	5,221	9,502	8,022	11,777	14,573	13,388
Loss (gain) on sales of assets		548	(2,253)	336			1,135	784	229	(1,917)	1,013	(1,369)	2,148	(1,124)	4,866	1,232
Loss on impairment				0					553					0	136,210	0
Restructuring charges			0					103		0	656	0	656	0	656	14,989
Derivative (gain) loss		5,442	(5,878)	2,600			5,815	6,178	(5,276)	(3,278)	902	2,164	6,717	(24,055)	5,587	31,100
Income from equity affiliates		(1,547)	(1,287)	(1,293)			(463)	(1,653)	(2,025)	(2,580)	(3,678)	(4,127)	(4,141)	(5,591)	(5,409)	(3,159)
Other operating loss (income)		3,368	3,659	(1,962)			(1,000)	0	2	1,697	0	5,065	(1,000)	8,309	(3,000)	181
Total costs, expenses and other		363,019	358,480	348,235			372,854	360,534	368,542	706,715	729,076	1,069,734	1,101,929	1,439,830	1,565,214	1,179,664
Operating income (loss)	$ 22,641	8,753	(883)	7,619	$ (121,618)		(20,854)	(8,797)	5,573	6,736	(3,221)	15,489	(24,075)	38,130	(145,696)	(48,664)
Interest expense		(30,882)	(30,860)	(28,927)			(25,865)	(24,850)	(23,999)	(59,787)	(48,849)	(90,669)	(74,714)	(121,819)	(101,311)	(82,320)
Loss on extinguishment of debt		0					(5,385)					0	(5,385)	0	(5,385)	(49,154)
Interest income		1,374	2,356	1,791			1,555	2,567	2,140	4,147	4,707	5,521	6,262	7,435	7,993	6,400
Gain (loss) on foreign exchange		220	(364)	(1,387)			1,679	(1,313)	2,109	(1,751)	795	(1,531)	2,474	(715)	3,674	(4,016)
Other income		303	254	(122)			356	534	193	132	726	435	1,082	38	1,740	1,031
Total other income (expense)		(28,985)	(28,614)	(28,645)			(27,660)	(23,062)	(19,557)	(57,259)	(42,621)	(86,244)	(70,281)	(115,061)	(93,289)	(128,059)
Loss before income taxes		(20,232)	(29,497)	(21,026)			(48,514)	(31,859)	(13,984)	(50,523)	(45,842)	(70,755)	(94,356)	(76,931)	(238,985)	(176,723)
Income tax expense (benefit)		(1,625)	(100)	(47,935)			4,362	7,556	2,040	(48,035)	9,596	(49,660)	13,958	(48,059)	(19,890)	23
Net loss		(18,607)	(29,397)	26,909			(52,876)	(39,415)	(16,024)	(2,488)	(55,438)	(21,095)	(108,314)	(28,872)	(219,095)	(176,746)
Less net loss attributable to noncontrolling interest		$ (239)	$ (808)	$ (498)			$ (1,458)	$ (1,246)	$ (2,146)	(1,306)	(3,392)	(1,545)	(4,850)	(1,771)	(5,453)	(921)
Net loss attributable to the Parent company										$ (1,182)	$ (52,046)	$ (19,550)	$ (103,464)	$ (27,101)	$ (213,642)	$ (175,825)
Basic and diluted (in dollars per share)	$ (0.41)	$ (0.99)	$ (1.54)	$ 1.50	$ (6.10)		$ (2.86)	$ (2.13)	$ (0.79)	$ (0.07)	$ (2.93)	$ (1.06)	$ (5.80)	$ (1.47)	$ (11.93)	$ (11.08)
Basic and diluted (in shares)		18,570	18,540				17,986	17,926				18,458	17,846	18,486	17,905	15,941
Basic (in shares)				18,262					17,621	18,401	17,775
Diluted (in shares)				18,269					17,621	18,418	17,775
As Reported [Member]
Revenues		$ 370,683	$ 356,247	$ 354,721			$ 349,796	$ 348,959	$ 371,483	$ 710,968	$ 720,444	$ 1,081,651	$ 1,070,240		$ 1,411,048	$ 1,115,992
Cost of sales		278,765	290,113	273,802			292,973	285,480	301,711	563,915	587,189	842,680	880,162		1,145,443	899,930
Depreciation, depletion and amortization		33,112	33,663	35,013			34,459	34,263	38,059	68,676	72,322	101,788	106,781		131,491	100,778
Selling and administrative		30,518	32,019	31,672			29,383	28,508	26,716	63,691	55,228	94,209	84,611		112,972	100,528
Heritage health benefit expenses		3,265	3,222	3,015			2,801	2,162	3,059	6,237	5,221	9,502	8,022		14,573	13,388
Loss (gain) on sales of assets		548	(2,253)	336			1,135	784	229	(1,917)	1,013	(1,369)	2,148		4,866	1,232
Loss on impairment				0					553						136,210	0
Restructuring charges			0					103		0	656	0	656		656	14,989
Derivative (gain) loss		5,442	(5,878)	2,600			5,815	6,178	(5,276)	(3,278)	902	2,164	6,717		5,587	31,100
Income from equity affiliates		(1,547)	(1,287)	(1,293)			(463)	(1,653)	(2,025)	(2,580)	(3,678)	(4,127)	(4,141)		(5,409)	(3,159)
Other operating loss (income)		3,368	3,659	(1,962)			(1,000)	0	2	1,697	0	5,065	(1,000)		(3,000)	181
Total costs, expenses and other		353,471	353,258	343,183			365,103	355,825	363,028	696,441	718,853	1,049,912	1,083,956		1,543,389	1,158,967
Operating income (loss)		17,212	2,989	11,538			(15,307)	(6,866)	8,455	14,527	1,591	31,739	(13,716)		(132,341)	(42,975)
Interest expense		(29,494)	(31,510)	(29,669)			(26,831)	(25,304)	(24,735)	(61,179)	(50,039)	(90,673)	(76,870)		(104,215)	(84,234)
Loss on extinguishment of debt		0					(5,385)					0	(5,385)		(5,385)	(49,154)
Interest income		1,374	2,356	1,791			1,555	2,567	2,140	4,147	4,707	5,521	6,262		7,993	6,400
Gain (loss) on foreign exchange		220	(364)	(1,387)			1,679	(1,313)	2,109	(1,751)	795	(1,531)	2,474		3,674	(4,016)
Other income		303	254	(122)			356	534	193	132	726	435	1,082		1,740	1,031
Total other income (expense)		(27,597)	(29,264)	(29,387)			(28,626)	(23,516)	(20,293)	(58,651)	(43,811)	(86,248)	(72,437)		(96,193)	(129,973)
Loss before income taxes		(10,385)	(26,275)	(17,849)			(43,933)	(30,382)	(11,838)	(44,124)	(42,220)	(54,509)	(86,153)		(228,534)	(172,948)
Income tax expense (benefit)		(1,625)	(100)	(47,935)			4,087	7,469	2,040	(48,035)	9,509	(49,660)	13,596		(19,767)	232
Net loss		(8,760)	(26,175)	30,086			(48,020)	(37,851)	(13,878)	3,911	(51,729)	(4,849)	(99,749)		(208,767)	(173,180)
Less net loss attributable to noncontrolling interest		(239)	(808)	(498)			(1,458)	(1,246)	(2,146)	(1,306)	(3,392)	(1,545)	(4,850)		(5,453)	(921)
Net loss attributable to the Parent company		$ (8,521)	$ (25,367)	$ 30,584			$ (46,562)	$ (36,605)	$ (11,732)	$ 5,217	$ (48,337)	$ (3,304)	$ (94,899)		$ (203,314)	$ (172,259)
Basic and diluted (in dollars per share)		$ (0.46)	$ (1.37)	$ 1.67			$ (2.59)	$ (2.04)	$ (0.67)	$ 0.28	$ (2.72)	$ (0.18)	$ (5.32)		$ (11.36)	$ (10.86)
Basic and diluted (in shares)		18,570	18,540				17,986	17,926				18,458	17,846		17,905	15,941
Basic (in shares)				18,262					17,621	18,401	17,775
Diluted (in shares)				18,269					17,621	18,418	17,775
Adjustments [Member]
Revenues		$ 1,089	$ 1,350	$ 1,133			$ 2,204	$ 2,778	$ 2,632	$ 2,483	$ 5,411	$ 3,572	$ 7,614		$ 8,470	$ 15,008
Cost of sales		6,663	8,068	7,323			9,296	7,224	8,001	15,392	15,225	22,055	24,520		30,406	30,338
Depreciation, depletion and amortization		7,748	1,560	2,002			2,781	2,069	1,849	3,561	3,918	11,309	6,699		8,837	8,583
Selling and administrative		(4,863)	(4,406)	(4,273)			(4,326)	(4,584)	(4,336)	(8,679)	(8,920)	(13,542)	(13,246)		(17,418)	(18,224)
Heritage health benefit expenses		0	0	0			0	0	0	0	0	0	0		0	0
Loss (gain) on sales of assets		0	0	0			0	0	0	0	0	0	0		0	0
Loss on impairment				0					0						0	0
Restructuring charges			0					0		0	0	0	0		0	0
Derivative (gain) loss		0	0	0			0	0	0	0	0	0	0		0	0
Income from equity affiliates		0	0	0			0	0	0	0	0	0	0		0	0
Other operating loss (income)		0	0	0			0	0	0	0	0	0	0		0	0
Total costs, expenses and other		9,548	5,222	5,052			7,751	4,709	5,514	10,274	10,223	19,822	17,973		21,825	20,697
Operating income (loss)		(8,459)	(3,872)	(3,919)			(5,547)	(1,931)	(2,882)	(7,791)	(4,812)	(16,250)	(10,359)		(13,355)	(5,689)
Interest expense		(1,388)	650	742			966	454	736	1,392	1,190	4	2,156		2,904	1,914
Loss on extinguishment of debt		0					0					0	0		0	0
Interest income		0	0	0			0	0	0	0	0	0	0		0	0
Gain (loss) on foreign exchange		0	0	0			0	0	0	0	0	0	0		0	0
Other income		0	0	0			0	0	0	0	0	0	0		0	0
Total other income (expense)		(1,388)	650	742			966	454	736	1,392	1,190	4	2,156		2,904	1,914
Loss before income taxes		(9,847)	(3,222)	(3,177)			(4,581)	(1,477)	(2,146)	(6,399)	(3,622)	(16,246)	(8,203)		(10,451)	(3,775)
Income tax expense (benefit)		0	0	0			275	87	0	0	87	0	362		(123)	(209)
Net loss		(9,847)	(3,222)	(3,177)			(4,856)	(1,564)	(2,146)	(6,399)	(3,709)	(16,246)	(8,565)		(10,328)	(3,566)
Less net loss attributable to noncontrolling interest			0	0				0	0	0	0	0	0		0	0
Net loss attributable to the Parent company		$ (9,847)	$ (3,222)	$ (3,177)			$ (4,856)	$ (1,564)	$ (2,146)	$ (6,399)	$ (3,709)	$ (16,246)	$ (8,565)		$ (10,328)	$ (3,566)
Basic and diluted (in dollars per share)		$ (0.53)	$ (0.17)	$ (0.17)			$ (0.27)	$ (0.09)	$ (0.12)	$ (0.35)	$ (0.21)	$ (0.88)	$ (0.48)		$ (0.57)	$ (0.22)
Basic and diluted (in shares)

[1] Operating loss for the three months ended December 31, 2015 includes $136.2 million of impairment charges at ROVA and the Coal Valley mine in the Coal - Canada segment.