BUSINESS SEGMENT INFORMATION - Summarized Financial Information by Segment (Detail) (USD $) In Thousands, unless otherwise specified	3 Months Ended				9 Months Ended
	Sep. 30, 2014		Sep. 30, 2013		Sep. 30, 2014 segments		Sep. 30, 2013		Dec. 31, 2013
Segment Reporting Information [Line Items]
Number of Reportable Segments					5
Revenues	$ 337,830		$ 176,792		$ 805,989		$ 500,739
Restructuring charges	3,265		0		11,207		0
Depreciation, depletion and amortization	28,175		17,434		68,713		47,257
Income (loss) from operations	(29,432)		8,536		(51,021)		26,245
Total assets	1,578,508		939,839		1,578,508		939,839		946,685
Capital expenditures	18,285		6,756		35,646		20,223
Coal - U.S. Segment
Segment Reporting Information [Line Items]
Revenues	165,051		151,881		473,350		433,330
Restructuring charges	292				783
Depreciation, depletion and amortization	13,771		14,838		40,552		39,496
Income (loss) from operations	4,793		9,564		17,069		35,130
Total assets	665,292		714,841		665,292		714,841
Capital expenditures	12,841		6,803		22,427		19,022
Coal - Canada Segment
Segment Reporting Information [Line Items]
Revenues	151,340	[1]	0	[1]	267,387	[1]	0	[1]
Restructuring charges	2,411	[1]			9,058	[1]
Depreciation, depletion and amortization	11,862	[1]	0	[1]	20,533	[1]	0	[1]
Income (loss) from operations	(1,453)	[1]	0	[1]	(14,162)	[1]	0	[1]
Total assets	607,565	[1]	0	[1]	607,565	[1]	0	[1]
Capital expenditures	5,280	[1]	0	[1]	12,775	[1]	0	[1]
Power Segment
Segment Reporting Information [Line Items]
Revenues	21,439		24,911		65,252		67,409
Restructuring charges	(40)				459
Depreciation, depletion and amortization	2,504		2,552		7,512		7,632
Income (loss) from operations	(26,413)		5,087		(34,147)		8,922
Total assets	163,528		180,967		163,528		180,967
Capital expenditures	164		17		419		715
Heritage Segment
Segment Reporting Information [Line Items]
Revenues	0		0		0		0
Restructuring charges	0				0
Depreciation, depletion and amortization	0		0		0		0
Income (loss) from operations	(3,915)		(4,326)		(11,303)		(12,031)
Total assets	15,996		15,419		15,996		15,419
Capital expenditures	0		0		0		0
Corporate Segment
Segment Reporting Information [Line Items]
Revenues	0		0		0		0
Restructuring charges	602				907
Depreciation, depletion and amortization	38		44		116		129
Income (loss) from operations	(2,444)		(1,789)		(8,478)		(5,776)
Total assets	126,127		28,612		126,127		28,612
Capital expenditures	$ 0		$ (64)		$ 25		$ 486

[1] 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 and there is no activity for 2013.