SEGMENT INFORMATION - Summarized Financial Information by Segment (Detail) $ in Thousands			3 Months Ended
		Aug. 01, 2015	Mar. 31, 2016 USD ($) segments		Mar. 31, 2015 USD ($)	Dec. 31, 2015 USD ($)
Segment Reporting Information [Line Items]
Number of Reportable Segments | segments			6
Revenues			$ 354,721		$ 371,483
Restructuring charges			0		553
Depreciation, depletion and amortization			35,013		38,059
Operating income (loss)			11,538		8,455
Total assets			1,770,689		1,815,052	$ 1,476,577
Capital expenditures			5,548		13,027
Coal - U.S. Segment
Segment Reporting Information [Line Items]
Revenues	[1]		155,179		154,869
Restructuring charges	[1]		0		0
Depreciation, depletion and amortization	[1]		14,138		9,379
Operating income (loss)	[1]		11,280		7,118
Total assets	[1]		945,328		576,344
Capital expenditures	[1]		2,655		7,157
Coal - Canada Segment
Segment Reporting Information [Line Items]
Revenues			93,434		103,242
Restructuring charges			0		0
Depreciation, depletion and amortization			5,639		11,265
Operating income (loss)			12,409		9,865
Total assets			523,876		583,536
Capital expenditures			1,349		4,049
Coal - WMLP Segment
Segment Reporting Information [Line Items]
Revenues	[2]		92,481		109,090
Restructuring charges	[2]		0		553
Depreciation, depletion and amortization	[2]		15,265		14,889
Operating income (loss)	[2]		809		(369)
Total assets	[2]		411,656		453,620
Capital expenditures	[2]		1,544		4,812
Power Segment
Segment Reporting Information [Line Items]
Revenues			21,995		20,647
Restructuring charges			0		0
Depreciation, depletion and amortization			0		2,484
Operating income (loss)			(5,801)		413
Total assets			41,717	[2]	172,310
Capital expenditures			0		579
Asset impairment charges			133,100		0
Heritage Segment
Segment Reporting Information [Line Items]
Revenues			0		0
Restructuring charges			0		0
Depreciation, depletion and amortization			0		0
Operating income (loss)			(3,481)		(3,349)
Total assets			16,379		16,005
Capital expenditures			0		0
Corporate and Eliminations
Segment Reporting Information [Line Items]
Revenues	[3]		(8,368)		(16,365)
Restructuring charges	[3]		0		0
Depreciation, depletion and amortization	[3]		(29)		42
Operating income (loss)	[3]		(3,678)		(5,223)
Total assets	[3]		(168,267)		13,237
Capital expenditures	[3]		0		(3,570)
Buckingham Coal Company, LLC | Coal - U.S. Segment
Segment Reporting Information [Line Items]
Revenues			26,600
Operating income (loss)			3,100
Revenues to the Coal - U.S. segment | Coal - WMLP Segment
Segment Reporting Information [Line Items]
Revenues			$ 7,900		$ 16,400
Westmoreland Kemmerer, LLC
Segment Reporting Information [Line Items]
Ownership interest in LLC (percent)		100.00%

[1] The San Juan Acquisition was completed on January 31, 2016. For the three months ended March 31, 2016, revenues for the San Juan Entities were $26.6 million and operating losses were $3.1 million.

[2] Total assets as of March 31, 2016 reflect a $133.1 million asset impairment in the Power segment that was recorded during the fourth quarter of 2015. No such impairment had been recorded as of March 31, 2015.

[3] The Coal - WMLP segment recorded revenues of $7.9 million and $16.4 million for intersegment revenues to the Coal - U.S. segment for the three months ended March 31, 2016 and March 31, 2015, respectively