Amount of currency on hand as well as demand deposits with banks or financial institutions. Includes other kinds of accounts that have the general characteristics of demand deposits. Also includes short-term, highly liquid investments that are both readily convertible to known amounts of cash and so near their maturity that they present insignificant risk of changes in value because of changes in interest rates. Excludes cash and cash equivalents within disposal group and discontinued operation.

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 210
-SubTopic 10
-Section 45
-Paragraph 1
-Subparagraph (a)
-URI http://asc.fasb.org/extlink&oid=82887183&loc=d3e6676-107765

Reference 2: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 230
-SubTopic 10
-Section 45
-Paragraph 4
-URI http://asc.fasb.org/extlink&oid=98514028&loc=d3e3044-108585

Reference 3: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 210
-SubTopic 10
-Section S99
-Paragraph 1
-Subparagraph (SX 210.5-02.1)
-URI http://asc.fasb.org/extlink&oid=6877327&loc=d3e13212-122682

+ Details

Name:	us-gaap_CashAndCashEquivalentsAtCarryingValue
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	instant

- Definition

Sum of the carrying values as of the balance sheet date of all debt, including all short-term borrowings, long-term debt, and capital lease obligations.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 942
-SubTopic 210
-Section S99
-Paragraph 1
-Subparagraph (SX 210.9-03.13,16)
-URI http://asc.fasb.org/extlink&oid=6876686&loc=d3e534808-122878

Reference 2: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 944
-SubTopic 210
-Section S99
-Paragraph 1
-Subparagraph (SX 210.7-03.16)
-URI http://asc.fasb.org/extlink&oid=6879938&loc=d3e572229-122910

+ Details

Name:	us-gaap_DebtAndCapitalLeaseObligations
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	instant

- Definition

Face (par) amount of debt instrument at time of issuance.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 835
-SubTopic 30
-Section 55
-Paragraph 8
-URI http://asc.fasb.org/extlink&oid=64930739&loc=d3e28878-108400

Reference 2: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 835
-SubTopic 30
-Section 45
-Paragraph 2
-URI http://asc.fasb.org/extlink&oid=64930785&loc=d3e28551-108399

+ Details

Name:	us-gaap_DebtInstrumentFaceAmount
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	instant

- Definition

Amount of unused borrowing capacity under the long-term financing arrangement that is available to the entity as of the balance sheet date.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 210
-SubTopic 10
-Section S99
-Paragraph 1
-Subparagraph (SX 210.5-02.22(b))
-URI http://asc.fasb.org/extlink&oid=6877327&loc=d3e13212-122682

+ Details

Name:	us-gaap_DebtInstrumentUnusedBorrowingCapacityAmount
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	instant

- Definition

Gross amount of debt extinguished.

+ References

No definition available.

+ Details

Name:	us-gaap_ExtinguishmentOfDebtAmount
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	duration

- Definition

Line items represent financial concepts included in a table. These concepts are used to disclose reportable information associated with domain members defined in one or many axes to the table.

+ References

No definition available.

+ Details

Name:	us-gaap_RelatedPartyTransactionLineItems
Namespace Prefix:	us-gaap_
Data Type:	xbrli:stringItemType
Balance Type:	na
Period Type:	duration

- Definition

Forbearances of Debt Covenants and Restrictions from Lenders and Note Holders, Percent

+ References

No definition available.

+ Details

Name:	wlb_ForbearancesofDebtCovenantsandRestrictionsfromLendersandNoteHoldersPercent
Namespace Prefix:	wlb_
Data Type:	num:percentItemType
Balance Type:	na
Period Type:	duration

- Definition

Interest Payment Deferred

+ References

No definition available.

+ Details

Name:	wlb_InterestPaymentDeferred
Namespace Prefix:	wlb_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	duration

- Definition

Interest and Principal Payment Deferred

+ References

No definition available.

+ Details

Name:	wlb_InterestandPrincipalPaymentDeferred
Namespace Prefix:	wlb_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	duration

- Details

Name:	us-gaap_RelatedPartyTransactionsByRelatedPartyAxis=wlb_WestmorelandResourcePartnersLPMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_LongtermDebtTypeAxis=us-gaap_BridgeLoanMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_DebtInstrumentAxis=wlb_BridgeLoanAgreementMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_LongtermDebtTypeAxis=us-gaap_SeniorNotesMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_DebtInstrumentAxis=wlb_SeniorSecuredNotesDue2021Member
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_SubsequentEventTypeAxis=us-gaap_SubsequentEventMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

BASIS OF PRESENTATION - Additional Information (Details) - USD ($)				6 Months Ended
	Jul. 02, 2018	Jun. 29, 2018	May 21, 2018	Jun. 30, 2018	Dec. 31, 2017	Jun. 30, 2017	Dec. 22, 2014
Related Party Transaction [Line Items]
Cash and cash equivalents				$ 63,187,000	$ 103,247,000	$ 57,620,000
Forbearances of debt covenants and restrictions from lenders and note holders, percent included				75.00%
Bridge loan				$ 1,110,741,000	1,075,908,000
Proceeds used to wxtinguish in full the Company's Revolver and San Juan Loan			$ 48,500,000
WMLP
Related Party Transaction [Line Items]
Cash and cash equivalents				28,200,000
Bridge Loan \| Bridge Loan Agreement
Related Party Transaction [Line Items]
Bridge loan			110,000,000
Bridge loan			90,000,000	$ 90,000,000	$ 0
Bridge loan, unused borrowing capacity			$ 20,000,000
Senior Notes \| Senior Secured Notes Due 2021
Related Party Transaction [Line Items]
Interest and Principal Payment Deferred		$ 8,000,000
Bridge loan							$ 350,000,000.0
Subsequent Event \| Senior Notes \| Senior Secured Notes Due 2021
Related Party Transaction [Line Items]
Interest Payment Deferred	$ 15,300,000