BUSINESS SEGMENT INFORMATION - Summarized Financial Information by Segment (Detail) (USD $) In Thousands, unless otherwise specified	3 Months Ended
	Mar. 31, 2015 segments		Mar. 31, 2014		Dec. 31, 2014
Segment Reporting Information [Line Items]
Number of Reportable Segments	6
Revenues	$ 371,483		$ 180,202
Restructuring charges	553		397
Depreciation, depletion and amortization	38,059		16,059
Operating income (loss)	8,455		8,053
Total assets	1,829,748		1,407,147		1,829,578
Capital expenditures	13,027		3,050
Coal - U.S. Segment
Segment Reporting Information [Line Items]
Revenues	196,392	[1]	158,190	[1]
Restructuring charges	0	[1]	0	[1]
Depreciation, depletion and amortization	14,088	[1]	13,459	[1]
Operating income (loss)	11,266	[1]	13,447	[1]
Total assets	733,833	[1]	697,916	[1]
Capital expenditures	10,399	[1]	3,053	[1]
Coal - Canada Segment
Segment Reporting Information [Line Items]
Revenues	103,242	[2]	0	[2]
Restructuring charges	0	[2]	0	[2]
Depreciation, depletion and amortization	11,265	[2]	0	[2]
Operating income (loss)	9,865	[2]	0	[2]
Total assets	593,684	[2]	0	[2]
Capital expenditures	4,049	[2]	0	[2]
Coal - WMLP Segment
Segment Reporting Information [Line Items]
Revenues	67,567	[3]	0	[3]
Restructuring charges	553	[3]	0	[3]
Depreciation, depletion and amortization	10,180	[3]	0	[3]
Operating income (loss)	(4,517)	[3]	0	[3]
Total assets	296,131	[3]	0	[3]
Capital expenditures	1,570	[3]	0	[3]
Power Segment
Segment Reporting Information [Line Items]
Revenues	20,647		22,012
Restructuring charges	0		397
Depreciation, depletion and amortization	2,484		2,524
Operating income (loss)	413		1,744
Total assets	172,310		167,998
Capital expenditures	579		35
Heritage Segment
Segment Reporting Information [Line Items]
Revenues	0		0
Restructuring charges	0		0
Depreciation, depletion and amortization	0		0
Operating income (loss)	(3,349)		(3,830)
Total assets	16,005		15,622
Capital expenditures	0		0
Corporate and Eliminations
Segment Reporting Information [Line Items]
Revenues	(16,365)	[4]	0
Restructuring charges	0		0
Depreciation, depletion and amortization	42		76
Operating income (loss)	(5,223)		(3,308)
Total assets	17,785		525,611
Capital expenditures	(3,570)		(38)
Buckingham Coal Company, LLC | Coal - U.S. Segment
Segment Reporting Information [Line Items]
Revenues	30,100	[1]
Operating income (loss)	1,000	[1]
Revenues to the Coal - U.S. segment | Coal - WMLP Segment
Segment Reporting Information [Line Items]
Revenues	$ 16,400	[3]

[1] The Buckingham Acquisition was completed on January 1, 2015. For the three months ended March 31, 2015, revenues and operating income for Buckingham were $30.1 million and $1.0 million, respectively.

[2] The Canadian operations were acquired on April 28, 2014, therefore, there is no activity for the three months ended March 31, 2014.

[3] The operations reported under the segment Coal - WMLP were acquired on December 31, 2014, therefore, there is no activity for the three months ended March 31, 2014.

[4] The Coal - WMLP segment recorded revenues of $16.4 million for intersegment revenues to the Coal - U.S. segment.