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ASSET RETIREMENT OBLIGATIONS, CONTRACTUAL THIRD-PARTY RECLAMATION RECEIVABLES, AND RECLAMATION DEPOSITS - Summary of Asset Retirement Obligations, Contractual Third-Party Reclamation Receivables, and Reclamation Deposits (Detail) (USD $)
In Thousands, unless otherwise specified
Mar. 31, 2015
Dec. 31, 2014
Mar. 31, 2014
Dec. 31, 2013
Asset Retirement Obligation [Line Items]        
Asset Retirement Obligation $ 447,402us-gaap_AssetRetirementObligation $ 452,745us-gaap_AssetRetirementObligation $ 281,263us-gaap_AssetRetirementObligation $ 279,864us-gaap_AssetRetirementObligation
Contractual Third- Party Reclamation Receivable 115,275wlb_ContractualThirdPartyReclamationReceivablesCurrentAndNonCurrent      
Reclamation deposits 76,715us-gaap_AssetRetirementObligationLegallyRestrictedAssetsFairValue      
Coal - U.S. Segment        
Asset Retirement Obligation [Line Items]        
Asset Retirement Obligation 295,569us-gaap_AssetRetirementObligation
/ us-gaap_StatementBusinessSegmentsAxis
= wlb_CoalU.S.SegmentMember		      
Contractual Third- Party Reclamation Receivable 109,126wlb_ContractualThirdPartyReclamationReceivablesCurrentAndNonCurrent
/ us-gaap_StatementBusinessSegmentsAxis
= wlb_CoalU.S.SegmentMember		      
Reclamation deposits 76,715us-gaap_AssetRetirementObligationLegallyRestrictedAssetsFairValue
/ us-gaap_StatementBusinessSegmentsAxis
= wlb_CoalU.S.SegmentMember		      
Coal - Canada Segment        
Asset Retirement Obligation [Line Items]        
Asset Retirement Obligation 118,285us-gaap_AssetRetirementObligation
/ us-gaap_StatementBusinessSegmentsAxis
= wlb_CoalCanadaSegmentMember		      
Contractual Third- Party Reclamation Receivable 6,149wlb_ContractualThirdPartyReclamationReceivablesCurrentAndNonCurrent
/ us-gaap_StatementBusinessSegmentsAxis
= wlb_CoalCanadaSegmentMember		      
Reclamation deposits 0us-gaap_AssetRetirementObligationLegallyRestrictedAssetsFairValue
/ us-gaap_StatementBusinessSegmentsAxis
= wlb_CoalCanadaSegmentMember		      
Coal - WMLP Segment        
Asset Retirement Obligation [Line Items]        
Asset Retirement Obligation 32,569us-gaap_AssetRetirementObligation
/ us-gaap_StatementBusinessSegmentsAxis
= wlb_CoalWestmorelandResourcePartnersLPMember		      
Contractual Third- Party Reclamation Receivable 0wlb_ContractualThirdPartyReclamationReceivablesCurrentAndNonCurrent
/ us-gaap_StatementBusinessSegmentsAxis
= wlb_CoalWestmorelandResourcePartnersLPMember		      
Reclamation deposits 0us-gaap_AssetRetirementObligationLegallyRestrictedAssetsFairValue
/ us-gaap_StatementBusinessSegmentsAxis
= wlb_CoalWestmorelandResourcePartnersLPMember		      
Power Segment        
Asset Retirement Obligation [Line Items]        
Asset Retirement Obligation 979us-gaap_AssetRetirementObligation
/ us-gaap_StatementBusinessSegmentsAxis
= wlb_PowerMember		      
Contractual Third- Party Reclamation Receivable 0wlb_ContractualThirdPartyReclamationReceivablesCurrentAndNonCurrent
/ us-gaap_StatementBusinessSegmentsAxis
= wlb_PowerMember		      
Reclamation deposits $ 0us-gaap_AssetRetirementObligationLegallyRestrictedAssetsFairValue
/ us-gaap_StatementBusinessSegmentsAxis
= wlb_PowerMember