BUSINESS SEGMENT INFORMATION - Summarized Financial Information by Segment (Detail) $ in Thousands		3 Months Ended		9 Months Ended		12 Months Ended
		Sep. 30, 2015 USD ($)	Sep. 30, 2014 USD ($)	Sep. 30, 2015 USD ($) segments	Sep. 30, 2014 USD ($)	Dec. 31, 2014 USD ($)
Segment Reporting Information [Line Items]
Number of Reportable Segments | segments				6
Revenues		$ 349,796	$ 337,830	$ 1,070,240	$ 805,989
Restructuring charges		0	3,265	656	11,207	$ 15,000
Depreciation, depletion and amortization		34,459	28,175	106,781	68,713
Operating income (loss)		(15,307)	(29,432)	(13,716)	(51,021)
Total assets		1,728,222	1,578,508	1,728,222	1,578,508	$ 1,829,578
Capital expenditures		19,417	18,285	57,971	35,646
Coal - U.S. Segment
Segment Reporting Information [Line Items]
Revenues	[1]	132,018	123,134	419,505	348,417
Restructuring charges	[1]	0	292	0	783
Depreciation, depletion and amortization	[1]	9,524	9,464	28,199	28,029
Operating income (loss)	[1]	482	(349)	8,403	(3,356)
Total assets	[1]	557,643	477,946	557,643	477,946
Capital expenditures	[1]	7,047	7,981	18,908	17,898
Coal - Canada Segment
Segment Reporting Information [Line Items]
Revenues	[2]	107,752	151,340	317,157	267,387
Restructuring charges	[2]	0	2,411	0	9,058
Depreciation, depletion and amortization	[2]	7,023	11,862	26,899	20,533
Operating income (loss)	[2]	4,009	(1,453)	23,397	(14,162)
Total assets	[2]	547,368	607,565	547,368	607,565
Capital expenditures	[2]	7,485	5,280	21,413	12,775
Coal - WMLP Segment
Segment Reporting Information [Line Items]
Revenues	[3]	94,785	41,917	300,908	124,933
Restructuring charges	[3]	0	0	656	0
Depreciation, depletion and amortization	[3]	15,471	4,307	44,282	12,523
Operating income (loss)	[3]	(4,845)	5,142	(6,151)	20,425
Total assets	[3]	440,004	187,346	440,004	187,346
Capital expenditures	[3]	4,691	4,860	19,918	4,529
Power Segment
Segment Reporting Information [Line Items]
Revenues		22,017	21,439	64,001	65,252
Restructuring charges		0	(40)	0	459
Depreciation, depletion and amortization		2,470	2,504	7,430	7,512
Operating income (loss)		(7,976)	(26,413)	(16,594)	(34,147)
Total assets		172,182	163,528	172,182	163,528
Capital expenditures		198	164	1,305	419
Heritage Segment
Segment Reporting Information [Line Items]
Revenues		0	0	0	0
Restructuring charges		0	0	0	0
Depreciation, depletion and amortization		0	0	0	0
Operating income (loss)		(2,950)	(3,915)	(8,699)	(11,303)
Total assets		16,152	15,996	16,152	15,996
Capital expenditures		0	0	0	0
Corporate and Eliminations
Segment Reporting Information [Line Items]
Revenues	[4]	(6,776)	0	(31,331)	0
Restructuring charges	[4]	0	602	0	907
Depreciation, depletion and amortization	[4]	(29)	38	(29)	116
Operating income (loss)	[4]	(4,027)	(2,444)	(14,072)	(8,478)
Total assets	[4]	(5,127)	126,127	(5,127)	126,127
Capital expenditures	[4]	(4)	$ 0	(3,573)	$ 25
Buckingham Coal Company, LLC | Coal - U.S. Segment
Segment Reporting Information [Line Items]
Revenues		15,900		61,400
Operating income (loss)		2,000		3,400
Revenues to the Coal - U.S. segment | Coal - WMLP Segment
Segment Reporting Information [Line Items]
Revenues		$ 5,900		$ 24,600

[1] The Buckingham Acquisition was completed on January 1, 2015. For the three and nine months ended September 30, 2015, revenues for Buckingham were $15.9 million and $61.4 million and operating losses were $2.0 million and $3.4 million, respectively.

[2] The Canadian Operations were acquired on April 28, 2014, therefore, information for the nine months ended September 30, 2014 includes approximately five months of operations.

[3] The operations reported under the segment Coal - WMLP were acquired on December 31, 2014, therefore, there is no activity for the three and nine months ended September 30, 2014.

[4] The Coal - WMLP segment recorded revenues of $5.9 million and $24.6 million for intersegment revenues to the Coal - U.S. segment for the three and nine months ended September 30, 2015.