BUSINESS SEGMENT INFORMATION - Summarized Financial Information by Segment (Detail)		3 Months Ended									12 Months Ended
		Dec. 31, 2015 USD ($)	Sep. 30, 2015 USD ($)	Jun. 30, 2015 USD ($)	Mar. 31, 2015 USD ($)	Dec. 31, 2014 USD ($)		Sep. 30, 2014 USD ($)	Jun. 30, 2014 USD ($)	Mar. 31, 2014 USD ($)	Dec. 31, 2015 USD ($) segments	Dec. 31, 2014 USD ($)		Dec. 31, 2013 USD ($)
Segment Reporting Information [Line Items]
Number of Reportable Segments | segments											6
Revenues											$ 1,411,048,000	$ 1,115,992,000		$ 674,686,000
Restructuring charges											656,000	14,989,000		5,078,000
Depreciation, depletion, and amortization											131,491,000	100,778,000		67,231,000
Operating income (loss)		$ (118,623,000)	$ (15,307,000)	$ (6,866,000)	$ 8,455,000	$ 8,044,000		$ (29,432,000)	$ (29,640,000)	$ 8,053,000	(132,341,000)	(42,975,000)		25,362,000
Total assets		1,502,396,000				1,816,495,000					1,502,396,000	1,816,495,000		941,330,000
Capital expenditures											77,921,000	50,326,000		28,591,000
Coal Segment [Member]
Segment Reporting Information [Line Items]
Revenues	[1],[2]										544,172,000	471,567,000		414,652,000
Restructuring charges	[1],[2]										0	1,058,000		0
Depreciation, depletion, and amortization	[1],[2]										37,507,000	37,651,000		38,956,000
Operating income (loss)	[1],[2]										12,107,000	(5,078,000)		15,744,000
Total assets	[1],[2]	520,467,000				475,999,000					520,467,000	475,999,000		511,675,000
Capital expenditures	[1],[2]										25,193,000	23,108,000		21,045,000
Coal Canada Segment [Member]
Segment Reporting Information [Line Items]
Revenues	[3]										430,519,000	388,664,000		0
Restructuring charges	[3]										0	9,565,000		0
Depreciation, depletion, and amortization	[3]										29,629,000	36,068,000		0
Operating income (loss)	[3]										40,291,000	(2,670,000)		0
Total assets	[3]	506,139,000				626,620,000					506,139,000	626,620,000		0
Capital expenditures	[3]										27,658,000	19,147,000		0
Westmoreland Resource Partners LP [Member]
Segment Reporting Information [Line Items]
Revenues	[4],[5]										388,605,000	170,508,000	[1]	172,467,000	[1]
Restructuring charges	[4],[5]										656,000	2,783,000		0
Depreciation, depletion, and amortization	[4],[5]										54,504,000	16,912,000	[1]	17,742,000	[1]
Operating income (loss)	[1],[4],[5]										(5,211,000)	26,478,000		28,726,000
Total assets	[4],[5]	420,907,000				482,379,000	[1]				420,907,000	482,379,000	[1]	194,141,000	[1]
Capital expenditures	[4],[5]										27,296,000	7,489,000	[1]	6,019,000	[1]
Power [Member]
Segment Reporting Information [Line Items]
Revenues											84,423,000	85,253,000		87,567,000
Restructuring charges											0	459,000		5,078,000
Depreciation, depletion, and amortization											9,908,000	9,998,000		10,179,000
Operating income (loss)											(146,868,000)	(35,023,000)		4,908,000
Total assets		39,762,000				172,104,000					39,762,000	172,104,000		180,684,000
Capital expenditures											1,408,000	527,000		790,000
Heritage [Member]
Segment Reporting Information [Line Items]
Revenues											0	0		0
Restructuring charges											0	78,000		0
Depreciation, depletion, and amortization											0	0		0
Operating income (loss)											(15,596,000)	(14,858,000)		(14,498,000)
Total assets		16,146,000				15,969,000					16,146,000	15,969,000		15,497,000
Capital expenditures											0	0		0
Corporate [Member]
Segment Reporting Information [Line Items]
Revenues	[6]										(36,671,000)	0		0
Restructuring charges	[6]										0	1,046,000		0
Depreciation, depletion, and amortization	[6]										(57,000)	149,000		354,000
Operating income (loss)	[6]										(17,064,000)	(11,824,000)		(9,518,000)
Total assets	[6]	$ (1,025,000)				$ 43,424,000					(1,025,000)	43,424,000		39,333,000
Capital expenditures	[6]										(3,634,000)	55,000		$ 737,000
Buckingham Coal Company, LLC [Member]
Segment Reporting Information [Line Items]
Revenues											80,400,000
Operating income (loss)											3,200,000
Ohio Operations [Member]
Segment Reporting Information [Line Items]
Revenues												225,200,000
Operating income (loss)												$ 26,800,000
Intersegment Eliminations [Member]
Segment Reporting Information [Line Items]
Revenues											$ 30,900,000

[1] Financial information of the Kemmerer mine for the year ended December 31, 2014 and 2013 was previously presented under the Coal - U.S. segment and is now presented under the Coal - WMLP segment for all periods presented due to the Kemmerer Drop that occurred on August 1, 2015.

[2] The Buckingham Acquisition was completed on January 1, 2015. For the year ended December 31, 2015, revenues for Buckingham were $80.4 million and operating losses were $3.2 million.

[3] The Canadian operations were acquired on April 28, 2014, therefore, information for the year ended December 31, 2014 includes approximately eight months of operations and there is no activity for 2013.

[4] The Coal - WMLP segment recorded revenues of $30.9 million for intersegment revenues to the Coal - U.S. segment for the year ended December 31, 2015.

[5] The Ohio operations reported under the segment Coal - WMLP were acquired on December 31, 2014. For the year ended December 31, 2015, revenues for the Ohio operations were $225.2 million and operating losses were $26.8 million.

[6] Eliminations for intersegment revenues and cost of sales are presented within the Corporate segment.