BUSINESS SEGMENT INFORMATION - Summarized Financial Information by Segment (Detail) (USD $) In Thousands, unless otherwise specified	3 Months Ended
	Mar. 31, 2013 segments	Mar. 31, 2012	Dec. 31, 2012
Segment Reporting Information [Line Items]
Number of Reportable Segments	4
Revenues	$ 161,448	$ 147,236
Operating income (loss)	5,734	9,085
Depreciation, depletion, and amortization	14,426	13,289
Total assets	943,015	955,002	936,115
Capital expenditures	5,301	4,420
Coal Segment
Segment Reporting Information [Line Items]
Revenues	142,112	126,514
Operating income (loss)	13,472	14,437
Depreciation, depletion, and amortization	11,801	10,682
Total assets	711,128	698,592
Capital expenditures	5,010	3,936
Power Segment
Segment Reporting Information [Line Items]
Revenues	19,336	20,722
Operating income (loss)	(1,003)	2,789
Depreciation, depletion, and amortization	2,531	2,499
Total assets	186,309	195,123
Capital expenditures	134	511
Heritage Segment
Segment Reporting Information [Line Items]
Revenues	0	0
Operating income (loss)	(4,175)	(4,010)
Depreciation, depletion, and amortization	0	0
Total assets	15,877	14,248
Capital expenditures	0	0
Corporate Segment
Segment Reporting Information [Line Items]
Revenues	0	0
Operating income (loss)	(2,560)	(4,131)
Depreciation, depletion, and amortization	94	108
Total assets	29,701	47,039
Capital expenditures	$ 157	$ (27)