LINES OF CREDIT AND DEBT - Additional Information (Detail) - USD ($)			9 Months Ended
	Aug. 01, 2015	Jan. 22, 2015	Sep. 30, 2015	Jun. 02, 2015	Dec. 31, 2014
Debt Instrument [Line Items]
Debt outstanding			$ 1,052,954,000		$ 984,787,000
Aggregate principal amount			1,064,473,000
Revolving lines of credit			0		9,576,000
Capital lease obligations incurred			15,000,000
Revolving line of credit
Debt Instrument [Line Items]
Maximum borrowing capacity				$ 50,000,000.0
Revolving lines of credit			0
Outstanding letters of credit			21,100,000
Senior secured notes due 2022 | Senior Notes
Debt Instrument [Line Items]
Debt outstanding			$ 350,000,000		350,000,000
Stated interest rate (percent)			8.75%
Term loan facility due 2020 | Term Notes
Debt Instrument [Line Items]
Debt outstanding			$ 327,994,000		350,000,000
Additional financing obtained per the Amended Financing Agreement		$ 75,000,000.0
Aggregate principal amount		425,000,000
Proceeds from term loan		$ 71,000,000
Discount rate used for issuance of add-on term loan (percent)		2.50%
Broker fee (percent)		1.50%
Consent fee (percent)		1.17%
Additional debt issuance cost		$ 100,000
WMLP term loan facility due 2018 | Term Notes
Debt Instrument [Line Items]
Debt outstanding			$ 297,414,000		$ 175,000,000
Seasonal Increase to Borrowing Capacity [Member] | Revolving line of credit
Debt Instrument [Line Items]
Maximum borrowing capacity				$ 75,000,000.0
Westmoreland Kemmerer, LLC | Term loan facility due 2020 | Term Notes
Debt Instrument [Line Items]
Repayments of debt	$ 94,100,000