BUSINESS SEGMENT INFORMATION - Summarized Financial Information by Segment (Detail) (USD $) In Thousands, unless otherwise specified	3 Months Ended								12 Months Ended
	Dec. 31, 2013	Sep. 30, 2013	Jun. 30, 2013	Mar. 31, 2013	Dec. 31, 2012	Sep. 30, 2012	Jun. 30, 2012	Mar. 31, 2012	Dec. 31, 2013 segments	Dec. 31, 2012	Dec. 31, 2011
Segment Reporting Information [Line Items]
Number of Reportable Segments									4
Revenues									$ 674,686	$ 600,437	$ 501,713
Restructuring charges									5,078	0	0
Operating income (loss)	(883)	8,536	11,975	5,734	8,597	15,451	(4,262)	9,086	25,362	28,872	10,626
Depreciation, depletion, and amortization									67,231	57,145	45,594
Total assets	946,685				936,115				946,685	936,115	759,172
Capital expenditures									28,591	21,032	27,594
Coal Segment [Member]
Segment Reporting Information [Line Items]
Revenues									587,119	519,152	414,928
Restructuring charges									0
Operating income (loss)									44,471	48,235	27,453
Depreciation, depletion, and amortization									56,698	46,639	35,112
Total assets	705,816				703,315				705,816	703,315	510,507
Capital expenditures									27,064	18,804	24,678
Power [Member]
Segment Reporting Information [Line Items]
Revenues									87,567	81,285	86,785
Restructuring charges									5,078
Operating income (loss)									4,907	8,244	12,119
Depreciation, depletion, and amortization									10,179	10,085	10,176
Total assets	180,684				189,599				180,684	189,599	194,730
Capital expenditures									790	2,070	2,119
Heritage [Member]
Segment Reporting Information [Line Items]
Revenues									0	0	0
Restructuring charges									0
Operating income (loss)									(14,498)	(14,711)	(19,675)
Depreciation, depletion, and amortization									0	0	0
Total assets	15,497				15,508				15,497	15,508	13,769
Capital expenditures									0	0	0
Corporate [Member]
Segment Reporting Information [Line Items]
Revenues									0	0	0
Restructuring charges									0
Operating income (loss)									(9,518)	(12,896)	(9,271)
Depreciation, depletion, and amortization									354	421	306
Total assets	44,688				27,693				44,688	27,693	40,166
Capital expenditures									$ 737	$ 158	$ 797