BUSINESS SEGMENT INFORMATION - Summarized Financial Information by Segment (Detail) (USD $) In Thousands, unless otherwise specified	3 Months Ended				6 Months Ended
	Jun. 30, 2014		Jun. 30, 2013		Jun. 30, 2014 segments		Jun. 30, 2013		Dec. 31, 2013
Segment Reporting Information [Line Items]
Number of Reportable Segments					5
Revenues	$ 287,956		$ 162,499		$ 468,159		$ 323,947
Restructuring charges	7,543		0		7,941		0
Depreciation, depletion and amortization	24,479		15,397		40,538		29,823
Income (loss) from operations	(29,640)		11,975		(21,588)		17,710
Total assets	1,583,721		933,603		1,583,721		933,603		946,685
Capital expenditures	14,311		8,166		17,361		13,467
Coal - U.S. Segment
Segment Reporting Information [Line Items]
Revenues	150,107		139,337		308,298		281,449
Restructuring charges	491				491
Depreciation, depletion and amortization	13,283		12,804		26,780		24,658
Income (loss) from operations	(423)		12,572		12,275		25,566
Total assets	670,375		699,556		670,375		699,556
Capital expenditures	6,533		7,209		9,586		12,219
Coal - Canada Segment
Segment Reporting Information [Line Items]
Revenues	116,046	[1]	0	[1]	116,046	[1]	0	[1]
Restructuring charges	6,646	[1]			6,646	[1]
Depreciation, depletion and amortization	8,671	[1]	0	[1]	8,671	[1]	0	[1]
Income (loss) from operations	(12,709)	[1]	0	[1]	(12,709)	[1]	0	[1]
Total assets	687,185	[1]	0	[1]	687,185	[1]	0	[1]
Capital expenditures	7,495	[1]	0	[1]	7,495	[1]	0	[1]
Power Segment
Segment Reporting Information [Line Items]
Revenues	21,803		23,162		43,815		42,498
Restructuring charges	101				499
Depreciation, depletion and amortization	2,486		2,551		5,009		5,081
Income (loss) from operations	(9,473)		4,837		(7,731)		3,836
Total assets	165,763		183,855		165,763		183,855
Capital expenditures	220		564		255		698
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	(3,559)		(3,530)		(7,389)		(7,705)
Total assets	15,664		16,305		15,664		16,305
Capital expenditures	0		0		0		0
Corporate Segment
Segment Reporting Information [Line Items]
Revenues	0		0		0		0
Restructuring charges	305				305
Depreciation, depletion and amortization	39		42		78		84
Income (loss) from operations	(3,476)		(1,904)		(6,034)		(3,987)
Total assets	44,734		33,887		44,734		33,887
Capital expenditures	$ 63		$ 393		$ 25		$ 550

[1] The Canadian operations were acquired on April 28, 2014, therefore, information for the three and six months ended June 30, 2014 includes approximately two months of operations and there is no activity for 2013.