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ASSET RETIREMENT OBLIGATIONS, CONTRACTUAL THIRD-PARTY RECLAMATION RECEIVABLES, AND RECLAMATION DEPOSITS - Cost Basis, Gross Unrealized Holding Gains and Fair Value of Available-for-Sale Securities (Detail) (Reclamation Deposits [Member], USD $)
In Thousands, unless otherwise specified
Dec. 31, 2014
Dec. 31, 2013
Reclamation Deposits [Member]
    
Schedule of Available-for-sale Securities [Line Items]    
Cost basis $ 32,930us-gaap_AvailableForSaleSecuritiesAmortizedCost
/ wlb_RestrictedSecuritiesHeldToSettleAssetRetirementObligationsAxis
= wlb_ReclamationDepositsMember		 $ 0us-gaap_AvailableForSaleSecuritiesAmortizedCost
/ wlb_RestrictedSecuritiesHeldToSettleAssetRetirementObligationsAxis
= wlb_ReclamationDepositsMember
Gross unrealized holding gains 886wlb_AvailableforsaleSecuritiesAccumulatedGrossUnrealizedGainbeforeTax
/ wlb_RestrictedSecuritiesHeldToSettleAssetRetirementObligationsAxis
= wlb_ReclamationDepositsMember		 0wlb_AvailableforsaleSecuritiesAccumulatedGrossUnrealizedGainbeforeTax
/ wlb_RestrictedSecuritiesHeldToSettleAssetRetirementObligationsAxis
= wlb_ReclamationDepositsMember
Gross unrealized holding losses (428)wlb_AvailableforsaleSecuritiesAccumulatedGrossUnrealizedLossbeforetax
/ wlb_RestrictedSecuritiesHeldToSettleAssetRetirementObligationsAxis
= wlb_ReclamationDepositsMember		 0wlb_AvailableforsaleSecuritiesAccumulatedGrossUnrealizedLossbeforetax
/ wlb_RestrictedSecuritiesHeldToSettleAssetRetirementObligationsAxis
= wlb_ReclamationDepositsMember
Fair Value $ 33,388us-gaap_AvailableForSaleSecurities
/ wlb_RestrictedSecuritiesHeldToSettleAssetRetirementObligationsAxis
= wlb_ReclamationDepositsMember		 $ 0us-gaap_AvailableForSaleSecurities
/ wlb_RestrictedSecuritiesHeldToSettleAssetRetirementObligationsAxis
= wlb_ReclamationDepositsMember