BUSINESS SEGMENT INFORMATION - Summarized Financial Information by Segment (Detail) (USD $) In Thousands, unless otherwise specified	3 Months Ended									12 Months Ended
	Dec. 31, 2014		Sep. 30, 2014	Jun. 30, 2014	Mar. 31, 2014	Dec. 31, 2013	Sep. 30, 2013	Jun. 30, 2013	Mar. 31, 2013	Dec. 31, 2014 segments		Dec. 31, 2013		Dec. 31, 2012
Segment Reporting Information [Line Items]
Number of Reportable Segments										6
Revenues										$ 1,115,992		$ 674,686		$ 600,437
Restructuring charges										14,989		5,078		0
Operating income (loss)	8,044		(29,432)	(29,640)	8,053	(883)	8,536	11,975	5,734	(42,975)		25,362		28,872
Depreciation, depletion, and amortization										100,778		67,231		57,145
Total assets	1,829,578					946,685				1,829,578		946,685		936,115
Capital expenditures										50,326		28,591		21,032
Coal Segment [Member]
Segment Reporting Information [Line Items]
Revenues										642,075		587,119		519,152
Restructuring charges										1,058		0
Operating income (loss)										24,183		44,471		48,235
Depreciation, depletion, and amortization										54,563		56,698		46,639
Total assets	650,626					705,816				650,626		705,816		703,315
Capital expenditures										30,597		27,064		18,804
Coal Canada Segment [Member]
Segment Reporting Information [Line Items]
Revenues										388,664	[1]
Restructuring charges										9,565	[1]	0	[1]
Operating income (loss)										(2,670)	[1]
Depreciation, depletion, and amortization										36,068	[1]
Total assets	635,155	[1]								635,155	[1]
Capital expenditures										19,147	[1]
Westmoreland Resource Partners LP [Member]
Segment Reporting Information [Line Items]
Revenues										0	[2]
Restructuring charges										2,783	[2]	0	[2]
Operating income (loss)										(2,783)	[2]
Depreciation, depletion, and amortization										0	[2]
Total assets	307,752	[2]								307,752	[2]
Capital expenditures										0	[2]
Power [Member]
Segment Reporting Information [Line Items]
Revenues										85,253		87,567		81,285
Restructuring charges										459		5,078
Operating income (loss)										(35,023)		4,907		8,244
Depreciation, depletion, and amortization										9,998		10,179		10,085
Total assets	172,104					180,684				172,104		180,684		189,599
Capital expenditures										527		790		2,070
Heritage [Member]
Segment Reporting Information [Line Items]
Revenues										0		0		0
Restructuring charges										78		0
Operating income (loss)										(14,858)		(14,498)		(14,711)
Depreciation, depletion, and amortization										0		0		0
Total assets	15,969					15,497				15,969		15,497		15,508
Capital expenditures										0		0		0
Corporate [Member]
Segment Reporting Information [Line Items]
Revenues										0		0		0
Restructuring charges										1,046		0
Operating income (loss)										(11,824)		(9,518)		(12,896)
Depreciation, depletion, and amortization										149		354		421
Total assets	47,972					44,688				47,972		44,688		27,693
Capital expenditures										$ 55		$ 737		$ 158

[1] 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 and 2012.

[2] The revenue from Customers B, C, D, E, F and G did not exceed 10% in 2014.