SEGMENT INFORMATION - Summarized Financial Information by Segment (Detail) $ in Thousands		3 Months Ended
		Mar. 31, 2017 USD ($) segments	Mar. 31, 2016 USD ($)	Dec. 31, 2016 USD ($)
Segment Reporting Information [Line Items]
Number of Reportable Segments | segments		6
Revenues		$ 339,737	$ 355,854
Depreciation, depletion and amortization		36,567	37,015
Operating income (loss)		(11,088)	7,619
Total assets		1,510,816	1,662,948	$ 1,584,909
Capital expenditures		7,210	5,548
Coal - U.S. Segment
Segment Reporting Information [Line Items]
Revenues	[1]	137,368	155,990
Depreciation, depletion and amortization	[1]	15,999	15,951
Operating income (loss)	[1]	4,336	7,667
Total assets	[1]	593,936	667,448
Capital expenditures	[1]	1,895	2,655
Coal - Canada Segment
Segment Reporting Information [Line Items]
Revenues		109,015	93,756
Depreciation, depletion and amortization		10,254	5,828
Operating income (loss)		(7,104)	12,103
Total assets		441,179	523,987
Capital expenditures		2,094	1,349
Coal - WMLP Segment
Segment Reporting Information [Line Items]
Revenues	[2]	74,805	92,481
Depreciation, depletion and amortization	[2]	10,351	15,265
Operating income (loss)	[2]	1,282	809
Total assets	[2]	375,177	411,656
Capital expenditures	[2]	3,221	1,544
Power Segment
Segment Reporting Information [Line Items]
Revenues		21,227	21,995
Depreciation, depletion and amortization		0	0
Operating income (loss)		(753)	(5,801)
Total assets		62,924	41,717
Capital expenditures		0	0
Heritage Segment
Segment Reporting Information [Line Items]
Revenues		0	0
Depreciation, depletion and amortization		0	0
Operating income (loss)		(3,670)	(3,481)
Total assets		16,446	16,379
Capital expenditures		0	0
Corporate and Eliminations
Segment Reporting Information [Line Items]
Revenues	[2]	(2,678)	(8,368)
Depreciation, depletion and amortization	[2]	(37)	(29)
Operating income (loss)	[2]	(5,179)	(3,678)
Total assets	[2]	21,154	1,761
Capital expenditures	[2]	$ 0	0
San Juan Coal Company | Coal Segment [Member]
Segment Reporting Information [Line Items]
Revenues			26,700
Operating income (loss)			$ 1,600

[1] The San Juan Acquisition was completed on January 31, 2016. For the three months ended March 31, 2016, revenues for the Westmoreland San Juan Entities were $26.7 million and operating losses were $1.6 million.

[2] The Coal - WMLP segment recorded revenues of $2.7 million for intersegment revenues to the Coal - U.S. segment for the three months ended March 31, 2017 and $8.4 million for the three months ended March 31, 2016. Eliminations for intersegment revenues and cost of sales are presented within the Corporate segment.