ACQUISITION - Summary of Pro Forma Information (Details) - USD ($) $ / shares in Units, $ in Thousands	3 Months Ended
Mar. 31, 2017	Mar. 31, 2016
Total revenues
As reported	$ 339,737	$ 355,854
Pro forma (unaudited)		382,129
Operating income
As reported	(11,088)	7,619
Pro forma (unaudited)		8,714
Net income applicable to common shareholders
As reported	$ (36,801)	27,407
Pro forma (unaudited)		$ 27,832
Net income per share applicable to common shareholders (basic and diluted)
As reported, (in dollars per share)	$ (1.98)	$ 1.50
Pro forma (unaudited) (in dollars per share)		$ 1.52

The pro forma net Income or Loss for the period as if the business combination or combinations had been completed at the beginning of a period.

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 805
-SubTopic 10
-Section 50
-Paragraph 2
-Subparagraph (h)(2)-(3)
-URI http://asc.fasb.org/extlink&oid=56956515&loc=d3e1392-128463

+ Details

Name:	us-gaap_BusinessAcquisitionsProFormaNetIncomeLoss
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	duration

- Definition

The pro forma revenue for a period as if the business combination or combinations had been completed at the beginning of the period.

+ References

+ Details

Name:	us-gaap_BusinessAcquisitionsProFormaRevenue
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	duration

- Definition

The amount of net income or loss for the period per each share in instances when basic and diluted earnings per share are the same amount and reported as a single line item on the face of the financial statements. Basic earnings per share is the amount of net income or loss for the period per each share of common stock or unit outstanding during the reporting period. Diluted earnings per share includes the amount of net income or loss for the period available to each share of common stock or common unit outstanding during the reporting period and to each share or unit that would have been outstanding assuming the issuance of common shares or units for all dilutive potential common shares or units outstanding during the reporting period.

+ References

No definition available.

+ Details

Name:	us-gaap_EarningsPerShareBasicAndDiluted
Namespace Prefix:	us-gaap_
Data Type:	num:perShareItemType
Balance Type:	na
Period Type:	duration

- Definition

Net income after adjustments for dividends on preferred stock (declared in the period) and/or cumulative preferred stock (accumulated for the period).

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 260
-SubTopic 10
-Section 45
-Paragraph 11
-URI http://asc.fasb.org/extlink&oid=7655603&loc=d3e1377-109256

+ Details

Name:	us-gaap_NetIncomeLossAvailableToCommonStockholdersBasic
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	duration

- Definition

The net result for the period of deducting operating expenses from operating revenues.

+ References

No definition available.

+ Details

Name:	us-gaap_OperatingIncomeLoss
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	duration

- Definition

Total revenue from sale of goods and services rendered during the reporting period, in the normal course of business, reduced by sales returns and allowances, and sales discounts.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 225
-SubTopic 10
-Section S99
-Paragraph 2
-Subparagraph (SX 210.5-03.1)
-URI http://asc.fasb.org/extlink&oid=26872669&loc=d3e20235-122688

+ Details

Name:	us-gaap_SalesRevenueNet
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	duration

- Definition

Business Acquisition, Pro Forma Earnings Per Share, Basic and Diluted

+ References

No definition available.

+ Details

Name:	wlb_BusinessAcquisitionProFormaEarningsPerShareBasicandDiluted
Namespace Prefix:	wlb_
Data Type:	num:perShareItemType
Balance Type:	na
Period Type:	duration

- Definition

Business Acquisition, Pro Forma Income (Loss) from Continuing Operations before Changes in Accounting and Extraordinary Items, Before Tax

+ References

No definition available.

+ Details

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

- Definition

Pro Forma Information Net Loss Applicable to Common Shareholders.

+ References

No definition available.

+ Details

Name:	wlb_ProFormaInformationNetLossApplicableToCommonShareholdersAbstract
Namespace Prefix:	wlb_
Data Type:	xbrli:stringItemType
Balance Type:	na
Period Type:	duration

- Definition

Pro Forma Information Net Loss Per Share Applicable to Common Shareholders.

+ References

No definition available.

+ Details

Name:	wlb_ProFormaInformationNetLossPerShareApplicableToCommonShareholdersAbstract
Namespace Prefix:	wlb_
Data Type:	xbrli:stringItemType
Balance Type:	na
Period Type:	duration

- Definition

Pro Forma Information Operating Income.

+ References

No definition available.

+ Details

Name:	wlb_ProFormaInformationOperatingIncomeAbstract
Namespace Prefix:	wlb_
Data Type:	xbrli:stringItemType
Balance Type:	na
Period Type:	duration

- Definition

Pro Forma Revenues [Abstract]

+ References

No definition available.

+ Details

Name:	wlb_ProFormaRevenuesAbstract
Namespace Prefix:	wlb_
Data Type:	xbrli:stringItemType
Balance Type:	na
Period Type:	duration