ACQUISITIONS - Narrative (Details)					1 Months Ended	8 Months Ended	9 Months Ended
	Jan. 01, 2015 USD ($)	Dec. 31, 2014 USD ($) shares	Jun. 25, 2014 USD ($)	Apr. 28, 2014 USD ($) mines	Jan. 31, 2015 USD ($)	Dec. 31, 2014 USD ($) shares	Sep. 30, 2015 USD ($)	Sep. 30, 2014 USD ($)
Business Acquisition [Line Items]
Cash paid							$ 35,887,000	$ 322,637,000
Buckingham Coal Company, LLC
Business Acquisition [Line Items]
Cash paid - Initial payment	$ 34,000,000
Cash received - working capital adjustment	1,600,000
Cash paid	32,400,000
Liabilities assumed	$ 10,800,000
Westmoreland Resource Partners LP
Business Acquisition [Line Items]
Units of partnership interest acquired (shares) | shares		4,512,500				4,512,500
Partnership interest, reverse split, conversion ratio		12
Westmoreland Resources GP, LLC
Business Acquisition [Line Items]
Cash paid		$ 30,000,000			$ 3,500,000
Goodwill		0				$ 0
Intangible assets		$ 31,000,000				31,000,000
Finite-Lived Intangible Asset, Useful Life		15 years
Liabilities assumed		$ 218,000,000				218,000,000
Canadian Subsidiaries
Business Acquisition [Line Items]
Cash paid - Initial payment				$ 282,800,000		282,800,000
Cash paid - working capital adjustment			$ 39,800,000			39,800,000
Cash paid						322,600,000
Goodwill		0				0
Intangible assets		37,000,000				37,000,000
Number of mines | mines				6
Voting interests acquired (percent)				50.00%
Liabilities assumed		$ 421,300,000		$ 421,300,000		$ 421,300,000
Acquisition-related costs								33,100,000
Indemnification asset				$ 27,900,000
Cost of Sales | Canadian Subsidiaries
Business Acquisition [Line Items]
Acquisition-related costs								14,200,000
Selling and Administrative Costs | Canadian Subsidiaries
Business Acquisition [Line Items]
Acquisition-related costs								7,800,000
Gain (Loss) on Foreign Exchange | Canadian Subsidiaries
Business Acquisition [Line Items]
Acquisition-related costs								6,200,000
Interest Expense | Canadian Subsidiaries
Business Acquisition [Line Items]
Acquisition-related costs								$ 4,900,000