BUSINESS SEGMENT INFORMATION - Summarized Financial Information by Segment (Detail) (USD $) In Thousands, unless otherwise specified	3 Months Ended		9 Months Ended
	Sep. 30, 2013	Sep. 30, 2012	Sep. 30, 2013 segments	Sep. 30, 2012	Dec. 31, 2012
Segment Reporting Information [Line Items]
Number of Reportable Segments			4
Revenues	$ 176,792	$ 161,332	$ 500,739	$ 441,410
Operating income (loss)	8,536	15,451	26,245	20,274
Depreciation, depletion and amortization	17,434	15,534	47,257	42,542
Total assets	939,839	971,153	939,839	971,153	936,115
Capital expenditures	6,756	4,085	20,223	16,066
Coal Segment
Segment Reporting Information [Line Items]
Revenues	151,881	138,798	433,330	382,272
Operating income (loss)	10,231	18,025	36,778	37,478
Depreciation, depletion and amortization	14,792	12,896	39,347	34,671
Total assets	714,841	719,129	714,841	719,129
Capital expenditures	6,803	3,872	19,022	13,910
Power Segment
Segment Reporting Information [Line Items]
Revenues	24,911	22,534	67,409	59,138
Operating income (loss)	5,087	4,023	8,922	5,066
Depreciation, depletion and amortization	2,551	2,534	7,632	7,554
Total assets	180,967	192,093	180,967	192,093
Capital expenditures	17	121	715	2,033
Heritage Segment
Segment Reporting Information [Line Items]
Revenues	0	0	0	0
Operating income (loss)	(4,326)	(4,149)	(12,031)	(12,687)
Depreciation, depletion and amortization	0	0	0	0
Total assets	15,419	15,077	15,419	15,077
Capital expenditures	0	0	0	0
Corporate Segment
Segment Reporting Information [Line Items]
Revenues	0	0	0	0
Operating income (loss)	(2,456)	(2,448)	(7,424)	(9,583)
Depreciation, depletion and amortization	91	104	278	317
Total assets	28,612	44,854	28,612	44,854
Capital expenditures	$ (64)	$ 92	$ 486	$ 123