QUARTERLY FINANCIAL DATA (UNAUDITED) (Details) - USD ($) $ / shares in Units, $ 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		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
Condensed Income Statements, Captions [Line Items]
Revenues		$ 392,737	$ 371,772	$ 357,597	$ 355,854	$ 341,666	[1]	$ 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
Operating income (loss)		22,641	8,753	(883)	7,619	(121,618)	[1]	(20,854)	(8,797)	5,573	$ 6,736	$ (3,221)	$ 15,489	$ (24,075)	38,130	(145,696)	(48,664)
Net Income (loss) applicable to common shareholders		$ (7,551)	$ (18,368)	$ (28,589)	$ 27,407	$ (110,180)	[1]	$ (51,418)	$ (38,169)	$ (13,878)					$ (27,101)	$ (213,645)	$ (176,684)
Basic and diluted (in dollars per share)		$ (0.41)	$ (0.99)	$ (1.54)	$ 1.50	$ (6.10)	[1]	$ (2.86)	$ (2.13)	$ (0.79)	$ (0.07)	$ (2.93)	$ (1.06)	$ (5.80)	$ (1.47)	$ (11.93)	$ (11.08)
Loss on impairment					$ 0					$ 553					$ 0	$ 136,210	$ 0
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
Coal Canada Segment [Member]
Condensed Income Statements, Captions [Line Items]
Operating income (loss)	[2]														39,104	36,830	(5,695)
Loss on impairment						$ 136,200										3,100
Depreciation, depletion and amortization	[2]														$ 26,893	$ 30,323	$ 36,450
Jewett Mine [Member]
Condensed Income Statements, Captions [Line Items]
Depreciation, depletion and amortization		$ 45,100

[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.

[2] The Canadian operations were acquired on April 28, 2014, therefore, information for the year ended December 31, 2014 includes approximately eight months of operations.