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ASSET RETIREMENT OBLIGATIONS, CONTRACTUAL THIRD-PARTY RECLAMATION RECEIVABLES, AND RECLAMATION DEPOSITS - Amortized Cost, Gross Unrealized Holding Gains and Losses and Fair Value of Held-to-Maturity Securities (Detail) (Reclamation Deposits [Member], USD $)
In Thousands, unless otherwise specified
Dec. 31, 2014
Dec. 31, 2013
Reclamation Deposits [Member]
    
Schedule of Held-to-maturity Securities [Line Items]    
Amortized cost $ 0us-gaap_HeldtomaturitySecuritiesRestrictedNoncurrent
/ wlb_RestrictedSecuritiesHeldToSettleAssetRetirementObligationsAxis
= wlb_ReclamationDepositsMember		 $ 11,396us-gaap_HeldtomaturitySecuritiesRestrictedNoncurrent
/ wlb_RestrictedSecuritiesHeldToSettleAssetRetirementObligationsAxis
= wlb_ReclamationDepositsMember
Gross unrealized holding gains 0wlb_HeldtomaturitySecuritiesAccumulatedUnrecognizedHoldingGain
/ wlb_RestrictedSecuritiesHeldToSettleAssetRetirementObligationsAxis
= wlb_ReclamationDepositsMember		 764wlb_HeldtomaturitySecuritiesAccumulatedUnrecognizedHoldingGain
/ wlb_RestrictedSecuritiesHeldToSettleAssetRetirementObligationsAxis
= wlb_ReclamationDepositsMember
Gross unrealized holding losses 0wlb_HeldtomaturitySecuritiesAccumulatedUnrecognizedHoldingLoss
/ wlb_RestrictedSecuritiesHeldToSettleAssetRetirementObligationsAxis
= wlb_ReclamationDepositsMember		 (74)wlb_HeldtomaturitySecuritiesAccumulatedUnrecognizedHoldingLoss
/ wlb_RestrictedSecuritiesHeldToSettleAssetRetirementObligationsAxis
= wlb_ReclamationDepositsMember
Fair value $ 0us-gaap_HeldToMaturitySecuritiesFairValue
/ wlb_RestrictedSecuritiesHeldToSettleAssetRetirementObligationsAxis
= wlb_ReclamationDepositsMember		 $ 12,086us-gaap_HeldToMaturitySecuritiesFairValue
/ wlb_RestrictedSecuritiesHeldToSettleAssetRetirementObligationsAxis
= wlb_ReclamationDepositsMember