ACQUISITIONS - Summary of purchase consideration and allocation of purchase consideration (Details)						1 Months Ended	3 Months Ended	12 Months Ended
	Aug. 01, 2015 USD ($)	Jan. 01, 2015 USD ($)	Dec. 31, 2014 USD ($) shares	Jun. 25, 2014 USD ($)	Apr. 28, 2014 USD ($) mines	Apr. 30, 2015 USD ($)	Sep. 30, 2014 USD ($)	Dec. 31, 2015 USD ($)	Dec. 31, 2014 USD ($) shares
Buckingham Coal Company, LLC [Member]
Business Acquisition [Line Items]
Cash purchase price		$ 32,500,000
Allocation of purchase consideration:
Land and mineral rights		12,100,000
Plant and equipment		26,800,000
Total Liabilities		$ (6,400,000)
Westmoreland Resource Partners LP [Member]
Business Acquisition [Line Items]
Units of partnership interest acquired (shares) | shares			4,512,500						4,512,500
Ownership percentage in partnership (percent)								93.80%	79.00%
Quarterly distributions (USD per unit)	$ 0.22					$ 0.20
Acquisition-related costs								$ 300,000	$ 4,500,000
Kemmerer Drop [Member]
Business Acquisition [Line Items]
Gain (loss) recognized on the Drop	0
Allocation of purchase consideration:
Net fair value	$ 102,600,000
Canadian Subsidiaries [Member]
Business Acquisition [Line Items]
Acquisition-related costs									33,600,000
Interest acquired (percent)					50.00%
Number of mines | mines					6
Purchase consideration:
Cash paid - Initial payment					$ 282,800,000
Cash paid - Working capital adjustment				$ 39,800,000
Allocation of purchase consideration:
Total Liabilities					(421,300,000)
Westmoreland Resources GP, LLC [Member]
Business Acquisition [Line Items]
Cash purchase price			$ 33,500,000
Goodwill			0						$ 0
Useful life of intangible assets									15 years
Partnership interest, conversion ratio	1
Purchase consideration:
Cash paid - Initial payment			30,000,000
Contingent consideration			3,500,000						$ 3,500,000
Fair value of outstanding WMLP units			10,800,000						10,800,000
Total purchase consideration			44,300,000
Allocation of purchase consideration:
Receivables			22,500,000						22,500,000
Inventories - materials and supplies			7,400,000						7,400,000
Inventories - coal			6,600,000						6,600,000
Other current assets			1,300,000						1,300,000
Total current assets			37,800,000						37,800,000
Land and mineral rights			40,200,000						40,200,000
Plant and equipment			134,000,000						134,000,000
Advanced coal royalties			9,200,000						9,200,000
Restricted investments and bond collateral			10,600,000						10,600,000
Intangible assets			31,000,000						31,000,000
Other assets			200,000						200,000
Total assets			263,000,000						263,000,000
Trade payables and other accrued liabilities			(19,100,000)						(19,100,000)
Asset retirement obligations			(7,800,000)						(7,800,000)
Other current liabilities			(4,000,000)						(4,000,000)
Total current liabilities			(30,900,000)						(30,900,000)
Long-term debt, less current installments			(160,100,000)						(160,100,000)
Asset retirement obligations, less current portion			(23,900,000)						(23,900,000)
Warrants			(2,000,000)						(2,000,000)
Other liabilities			(1,800,000)						(1,800,000)
Total Liabilities			(218,700,000)						(218,700,000)
Net fair value			44,300,000						44,300,000
Non-controlling Interest			(10,800,000)						(10,800,000)
Invested Equity			33,500,000						33,500,000
Canadian Acquisition [Member]
Business Acquisition [Line Items]
Cash purchase price			322,600,000
Repayment of assumed liability					18,100,000
Net cash received in the acquisition					8,100,000
Intangible asset transferred							$ 37,000,000
Proceeds from transfer of intangible asset							37,000,000
Gain (loss) on transfer of intangible asset							$ 0
Indemnification asset					$ 27,900,000
Purchase consideration:
Cash paid - Initial payment			282,800,000
Cash paid - Working capital adjustment			39,800,000
Allocation of purchase consideration:
Cash and cash equivalents			26,200,000						26,200,000
Receivables			78,100,000						78,100,000
Inventories - materials and supplies			52,000,000						52,000,000
Inventories - coal			79,800,000						79,800,000
Loan and lease receivables			11,200,000						11,200,000
Deferred tax assets			8,200,000						8,200,000
Other current assets			3,400,000						3,400,000
Total current assets			258,900,000						258,900,000
Land and mineral rights			202,600,000						202,600,000
Plant and equipment			114,800,000						114,800,000
Loan and lease receivables			79,100,000						79,100,000
Contractual third-party reclamation receivables, less current portion			6,800,000						6,800,000
Investment in joint venture			36,000,000						36,000,000
Intangible assets			37,000,000						37,000,000
Other assets			8,700,000						8,700,000
Total assets			743,900,000						743,900,000
Current installments of long-term debt			(36,300,000)						(36,300,000)
Trade payables and other accrued liabilities			(136,100,000)						(136,100,000)
Asset retirement obligations			(7,800,000)						(7,800,000)
Total current liabilities			(180,200,000)						(180,200,000)
Long-term debt, less current installments			(86,300,000)						(86,300,000)
Asset retirement obligations, less current portion			(122,900,000)						(122,900,000)
Deferred tax liabilities			(31,900,000)						(31,900,000)
Total Liabilities			(421,300,000)						(421,300,000)
Net fair value			$ 322,600,000						322,600,000
Cost of Sales [Member] | Canadian Subsidiaries [Member]
Business Acquisition [Line Items]
Acquisition-related costs									14,200,000
Selling, General and Administrative Expenses [Member] | Canadian Subsidiaries [Member]
Business Acquisition [Line Items]
Acquisition-related costs									8,300,000
Foreign Currency Gain (Loss) [Member] | Canadian Subsidiaries [Member]
Business Acquisition [Line Items]
Acquisition-related costs									6,200,000
Interest Expense [Member] | Canadian Subsidiaries [Member]
Business Acquisition [Line Items]
Acquisition-related costs									$ 4,900,000
Westmoreland Kemmerer, LLC [Member]
Business Acquisition [Line Items]
Ownership interest in LLC (percent)	100.00%
Aggregate consideration received from contribution of LLC	$ 230,000,000
Cash consideration received	115,000,000
WMLP Series A Convertible Units [Member] | Westmoreland Kemmerer, LLC [Member]
Business Acquisition [Line Items]
Partnership units received as consideration	$ 115,000,000