DEBT AND LINES OF CREDIT - Additional Information (Detail) - USD ($)						3 Months Ended		6 Months Ended		12 Months Ended
	May 22, 2018	May 21, 2018	Aug. 01, 2015	Jan. 22, 2015	Dec. 22, 2014	Jun. 30, 2018	Jun. 30, 2017	Jun. 30, 2018	Jun. 30, 2017	Dec. 31, 2017	Dec. 31, 2016	Jan. 31, 2016	Dec. 31, 2014	Dec. 16, 2014
Debt Instrument [Line Items]
Bridge loan						$ 1,110,741,000		$ 1,110,741,000		$ 1,075,908,000
Payments to Acquire Businesses, Gross								0	$ 3,580,000
Non-cash interest expense								4,782,000	4,639,000
Extinguishment of debt, amount		$ 48,500,000
Long-term Debt						1,110,741,000		1,110,741,000
Loss on extinguishment of debt						(1,838,000)	$ 0	(1,838,000)	$ 0
Revolving line of credit
Debt Instrument [Line Items]
Extinguishment of debt, amount		12,000,000
Loss on extinguishment of debt		1,200,000
Bridge Loan Agreement | Bridge Loan
Debt Instrument [Line Items]
Bridge loan		90,000,000				$ 90,000,000		$ 90,000,000		0
Bridge loan, unused borrowing capacity		$ 20,000,000
Equity in the holding company for the Company's Canadian business collateralizing debt (percent)		35.00%
Proceeds from Issuance of Debt		$ 84,000,000
Senior secured notes		110,000,000
Debt Issuance Costs, Gross		$ 2,700,000
Debt Instrument, Unamortized Discount, Percentage		3.33%
Senior Secured Notes Due 2021 | Senior Notes
Debt Instrument [Line Items]
Stated interest rate (percent)						8.75%		8.75%
Senior secured notes					$ 350,000,000.0
Discount Rate Used For Issuance Of Secured Notes														1.292%
Term loan facility due 2020 | Term Notes
Debt Instrument [Line Items]
Bridge loan						$ 319,773,000		$ 319,773,000		320,595,000
Stated interest rate (percent)						8.83%		8.83%
Senior secured notes				$ 75,000,000
Discount Rate Used For Issuance Of Secured Notes					2.50%
Long-term Debt				425,000,000
Proceeds from Issuance of Long-term Debt				$ 71,000,000
Debt Instrument, Unamortized Discount, Percentage				2.50%
Debt Issuance, Broker Fee, Percent				1.50%
Debt Issuance, Consent Fee, Percent				1.17%
Debt Issuance, Additional Cost				$ 100,000
Debt Instrument, Periodic Payment, Principal								$ 1,100,000
San Juan Loan | Term Notes
Debt Instrument [Line Items]
Bridge loan						$ 0		0		56,640,000
Senior secured notes												$ 125,000,000.0
Extinguishment of debt, amount	$ 50,600,000
Loss on extinguishment of debt	$ 600,000
WML Term Debt Due 2018
Debt Instrument [Line Items]
Maximum capacity to make distributions						$ 15,000,000		$ 15,000,000
Consolidated net leverage ratio (in excess at June 30,2017)						3.75		3.75
Fixed charge coverage ratio (below at June 30, 2017)						1.00		1.00
Minimum liquidity requirement						$ 7,500,000		$ 7,500,000
WML Term Debt Due 2018 | Term Notes
Debt Instrument [Line Items]
Bridge loan						$ 315,945,000		$ 315,945,000		$ 312,734,000
Senior secured notes													$ 175,000,000
Interest rate at period end (percent)						10.83%		10.83%
Paid-in-kind interest (percent)								3.00%		3.00%
Non-cash interest expense								$ 4,800,000		$ 4,600,000
Debt outstanding						$ 285,800,000		285,800,000
Accrued paid-in-kind interest						30,200,000		30,200,000
Line of Credit Facility, Option for Additional Term Loan			$ 120,000,000
Debt Issuance Costs, Gross													$ 8,600,000
Westmoreland Kemmerer, LLC
Debt Instrument [Line Items]
Proceeds from Kemmerer Drop used to pay down the term loan			$ 94,100,000
WMLP and subsidiaries
Debt Instrument [Line Items]
Long-term Debt						$ 327,684,000		$ 327,684,000
Federal Funds Rate | Bridge Loan Agreement | Bridge Loan
Debt Instrument [Line Items]
Basis spread on variable rate (percent)		0.50%
Federal Funds Rate | Senior Secured Notes Due 2021 | Senior Notes
Debt Instrument [Line Items]
Basis spread on variable rate (percent)								0.05%
Federal Funds Rate | Term loan facility due 2020 | Term Notes
Debt Instrument [Line Items]
Percentage added to the base rate before including the basis spread (percent)								0.05%
One, Two, Three, or Six Month LIBOR | Term loan facility due 2020 | Term Notes
Debt Instrument [Line Items]
Basis spread on variable rate (percent)					6.50%
London Interbank Offered Rate (LIBOR) | Bridge Loan Agreement | Bridge Loan
Debt Instrument [Line Items]
Stated interest rate (percent)		8.25%
Basis spread on variable rate (percent)		1.00%
Interest rate at period end (percent)		10.58%
London Interbank Offered Rate (LIBOR) | WML Term Debt Due 2018 | Term Notes
Debt Instrument [Line Items]
Basis spread on variable rate (percent)								8.50%
Debt instrument, floor interest rate (percent)						0.75%		0.75%
Interest rate at period end (percent)						2.33%		2.33%
Base Rate [Member] | Bridge Loan Agreement | Bridge Loan
Debt Instrument [Line Items]
Stated interest rate (percent)		7.25%
One-month LIBOR | Bridge Loan Agreement | Bridge Loan
Debt Instrument [Line Items]
Basis spread on variable rate (percent)		1.00%
One-month LIBOR | Senior Secured Notes Due 2021 | Senior Notes
Debt Instrument [Line Items]
Basis spread on variable rate (percent)								1.00%
One-month LIBOR | Term loan facility due 2020 | Term Notes
Debt Instrument [Line Items]
Basis spread on variable rate (percent)								6.50%
Percentage added to the base rate before including the basis spread (percent)								1.00%
Highest Rate [Member] | Senior Secured Notes Due 2021 | Senior Notes
Debt Instrument [Line Items]
Basis spread on variable rate (percent)								5.50%
Two-month LIBOR | Term loan facility due 2020 | Term Notes
Debt Instrument [Line Items]
Basis spread on variable rate (percent)								6.50%
Three-month LIBOR | Term loan facility due 2020 | Term Notes
Debt Instrument [Line Items]
Basis spread on variable rate (percent)								6.50%
Six-month LIBOR | Term loan facility due 2020 | Term Notes
Debt Instrument [Line Items]
Basis spread on variable rate (percent)								6.50%
Federal Funds Rate plus 0.05% | Term loan facility due 2020 | Term Notes
Debt Instrument [Line Items]
Basis spread on variable rate (percent)								5.50%
One-month LIBOR plus 1% | Term loan facility due 2020 | Term Notes
Debt Instrument [Line Items]
Basis spread on variable rate (percent)								5.50%
Minimum | WML Term Debt Due 2018 | Term Notes
Debt Instrument [Line Items]
Paid-in-kind interest (percent)								1.00%
Maximum | WML Term Debt Due 2018 | Term Notes
Debt Instrument [Line Items]
Paid-in-kind interest (percent)								3.00%
San Juan Coal Company
Debt Instrument [Line Items]
Payments to Acquire Businesses, Gross											$ 121,000,000