ACQUISITIONS - Summary of purchase consideration and allocation of purchase consideration (Details) - USD ($) $ in Thousands						1 Months Ended	8 Months Ended	9 Months Ended
		Jan. 01, 2015	Dec. 31, 2014	Jun. 25, 2014	Apr. 28, 2014	Jan. 31, 2015	Dec. 31, 2014	Sep. 30, 2015	Sep. 30, 2014
Purchase Price:
Cash paid								$ 35,887	$ 322,637
Buckingham Coal Company, LLC
Purchase Price:
Cash paid - Initial payment		$ 34,000
Cash received - Working capital adjustment		(1,600)
Cash paid		32,400
Assets:
Cash and cash equivalents		2,800
Inventories - materials and supplies		2,500
Other current assets		100
Total current assets		5,400
Land and mineral rights		13,200
Plant and equipment		24,600
Total Assets		43,200
Liabilities:
Trade payables and other accrued liabilities		(5,200)
Asset retirement obligations		(1,000)
Total current liabilities		(6,200)
Asset retirement obligations, less current portion		(2,800)
Other liabilities		(1,800)
Total Liabilities		(10,800)
Net fair value		$ 32,400
Westmoreland Resources GP, LLC
Purchase Price:
Cash paid			$ 30,000			$ 3,500
Contingent consideration			3,500				$ 3,500
Fair value of outstanding WMLP units	[1]		10,800				10,800
Total purchase consideration			44,300
Assets:
Trade receivables and other			22,500				22,500
Inventories - materials and supplies			7,400				7,400
Inventories - coal			6,600				6,600
Other current assets			1,300				1,300
Total current assets			37,800				37,800
Land and mineral rights			39,500				39,500
Plant and equipment			134,000				134,000
Advanced coal royalties			9,200				9,200
Restricted investments and bond collateral			10,600				10,600
Intangible assets			31,000				31,000
Other assets			200				200
Total Assets			262,300				262,300
Liabilities:
Trade payables and other accrued liabilities			(19,100)				(19,100)
Asset retirement obligations			(7,800)				(7,800)
Other current liabilities			(4,000)				(4,000)
Total current liabilities			(30,900)				(30,900)
Long-term debt, less current installments			(160,100)				(160,100)
Asset retirement obligations, less current portion			(23,900)				(23,900)
Warrants			(2,000)				(2,000)
Other liabilities			(1,100)				(1,100)
Total Liabilities			(218,000)				(218,000)
Net fair value			44,300				44,300
Non-controlling Interest			(10,800)				(10,800)
Invested Equity			33,500				33,500
Canadian Subsidiaries
Purchase Price:
Cash paid - Initial payment					$ 282,800		282,800
Cash paid - working capital adjustment				$ 39,800			39,800
Cash paid							322,600
Assets:
Cash and cash equivalents			26,200				26,200
Trade receivables and other			78,100				78,100
Inventories - materials and supplies			52,000				52,000
Inventories - coal			79,800				79,800
Loan and lease receivables			11,200				11,200
Deferred tax assets			8,200				8,200
Other current assets			3,400				3,400
Total current assets			258,900				258,900
Land and mineral rights			202,600				202,600
Plant and equipment			114,800				114,800
Loan and lease receivables			79,100				79,100
Contractual third-party reclamation receivables, less current portion			6,800				6,800
Investment in joint venture			36,000				36,000
Intangible assets			37,000				37,000
Other assets			8,700				8,700
Total Assets			743,900				743,900
Liabilities:
Current installments of long-term debt			(36,300)				(36,300)
Trade payables and other accrued liabilities			(136,100)				(136,100)
Asset retirement obligations			(7,800)				(7,800)
Total current liabilities			(180,200)				(180,200)
Long-term debt, less current installments			(86,300)				(86,300)
Asset retirement obligations, less current portion			(122,900)				(122,900)
Deferred tax liabilities			(31,900)				(31,900)
Total Liabilities			(421,300)		$ (421,300)		(421,300)
Net fair value			$ 322,600				$ 322,600

[1] Represents the market price of WMLP units outstanding using the December 31, 2014 closing price.