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RESTRICTED INVESTMENTS AND BOND COLLATERAL - Cost Basis, Gross Unrealized Holding Gains and Fair Value of Available-for-Sale Securities (Detail) (Restricted Investments and Bond Collateral [Member], USD $)
In Thousands, unless otherwise specified
Dec. 31, 2014
Dec. 31, 2013
Restricted Investments and Bond Collateral [Member]
    
Schedule of Available-for-sale Securities [Line Items]    
Cost basis $ 33,879us-gaap_AvailableForSaleSecuritiesAmortizedCost
/ us-gaap_InvestmentTypeAxis
= wlb_RestrictedInvestmentsandBondCollateralMember		 $ 0us-gaap_AvailableForSaleSecuritiesAmortizedCost
/ us-gaap_InvestmentTypeAxis
= wlb_RestrictedInvestmentsandBondCollateralMember
Gross unrealized holding gains 761wlb_AvailableforsaleSecuritiesAccumulatedGrossUnrealizedGainbeforeTax
/ us-gaap_InvestmentTypeAxis
= wlb_RestrictedInvestmentsandBondCollateralMember		 0wlb_AvailableforsaleSecuritiesAccumulatedGrossUnrealizedGainbeforeTax
/ us-gaap_InvestmentTypeAxis
= wlb_RestrictedInvestmentsandBondCollateralMember
Gross unrealized holding losses (583)wlb_AvailableforsaleSecuritiesAccumulatedGrossUnrealizedLossbeforetax
/ us-gaap_InvestmentTypeAxis
= wlb_RestrictedInvestmentsandBondCollateralMember		 0wlb_AvailableforsaleSecuritiesAccumulatedGrossUnrealizedLossbeforetax
/ us-gaap_InvestmentTypeAxis
= wlb_RestrictedInvestmentsandBondCollateralMember
Fair value $ 34,057us-gaap_AvailableForSaleSecurities
/ us-gaap_InvestmentTypeAxis
= wlb_RestrictedInvestmentsandBondCollateralMember