LINES OF CREDIT AND LONG-TERM DEBT - Additional Information (Detail) - USD ($)				12 Months Ended
	Aug. 01, 2015	Jan. 22, 2015	Dec. 16, 2014	Dec. 31, 2015	Dec. 31, 2014	Jul. 31, 2015	Jul. 30, 2015	Jun. 02, 2015	Mar. 31, 2014
Senior Notes:
Aggregate principal amount				$ 1,056,809,000
Capital leases entered into during the period				15,200,000
Revolving Credit Facility [Member]
Senior Notes:
Line of credit facility maximum borrowing capacity			$ 50,000,000						$ 60,000,000
Line of credit facility, remaining borrowing capacity				28,200,000
Letters of credit outstanding amount				19,800,000
Line of credit outstanding				$ 2,000,000
Debt issuance costs capitalized					$ 700,000
Unused line fee (percent)			0.50%
Capital Lease Obligations [Member]
Senior Notes:
Weighted average interest rate (percent)				4.51%	4.66%
Senior Secured Notes Due 2021 [Member] | Senior Notes [Member]
Senior Notes:
Additional financing obtained per the Amended Financing Agreement			$ 350,000,000.0
Discount rate used for issuance of notes (percent)			1.292%
Stated interest rate			8.75%
Capitalized debt issuance costs				$ 10,200,000
Redemption price as percent of principal (percent)				100.00%
Percentage of aggregate principal that may be redeemed (percent)				35.00%
Price, as a percentage of face value (percent)				108.75%
Term Loan Facility Due 2020 [Member] | Term Notes [Member]
Senior Notes:
Additional financing obtained per the Amended Financing Agreement		$ 75,000,000	$ 350,000,000.0
Discount rate used for issuance of notes (percent)			2.50%
Capitalized debt issuance costs					$ 8,400,000
Interest rate at year period end				7.50%
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
USD Denominated Sub-facility [Member] | Revolving Credit Facility [Member]
Senior Notes:
Line of credit facility maximum borrowing capacity			$ 30,000,000
CAD-Denominated Sub-facility [Member] | Revolving Credit Facility [Member]
Senior Notes:
Line of credit facility maximum borrowing capacity			$ 20,000,000
WMLP Term Debt Due 2018 [Member]
Senior Notes:
Restrictive covenant, maximum aggregate distribution						$ 15,000,000	$ 7,500,000
Restrictive covenant, minimum liquidity						$ 7,500,000	$ 5,000,000
WMLP Term Debt Due 2018 [Member] | Term Notes [Member]
Senior Notes:
Additional financing obtained per the Amended Financing Agreement					175,000,000
Interest rate at year period end				9.25%
Option for additional term loan					$ 120,000,000
Outstanding balance				$ 299,200,000
Fixed portion of the effective interest rate (percent)				8.50%
Paid-in-kind Interest				$ 6,900,000
Credit Facility Seasonal Increase to Borrowing Capacity [Member] | Revolving Credit Facility [Member]
Senior Notes:
Line of credit facility maximum borrowing capacity								$ 75,000,000.0
Base Rate [Member] | Revolving Credit Facility [Member]
Senior Notes:
Spread on basis, percentage			0.75%
LIBOR [Member] | Revolving Credit Facility [Member]
Senior Notes:
Spread on basis, percentage			2.75%
LIBOR [Member] | Term Loan Facility Due 2020 [Member] | Term Notes [Member]
Senior Notes:
Spread on basis, percentage			6.50%
LIBOR [Member] | WMLP Term Debt Due 2018 [Member] | Term Notes [Member]
Senior Notes:
Spread on basis, percentage				0.75%
Westmoreland Kemmerer, LLC [Member] | Term Loan Facility Due 2020 [Member] | Term Notes [Member]
Senior Notes:
Repayments of debt	$ 94,100,000
Minimum [Member] | WMLP Term Debt Due 2018 [Member] | Term Notes [Member]
Senior Notes:
Paid-in-kind interest (percent)				1.00%
Maximum [Member] | WMLP Term Debt Due 2018 [Member] | Term Notes [Member]
Senior Notes:
Paid-in-kind interest (percent)				3.00%