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SHAREHOLDERS' DEFICIT AND ACCUMULATED OTHER COMPREHENSIVE INCOME (LOSS) - Changes in Accumulated Other Comprehensive Income by component (Details) (USD $)
In Thousands, unless otherwise specified
3 Months Ended
Mar. 31, 2015
Accumulated Other Comprehensive Income (Loss) [Line Items]  
Balance at beginning of period $ (124,296)us-gaap_AccumulatedOtherComprehensiveIncomeLossNetOfTax
Other comprehensive income before reclassifications (27,201)us-gaap_OtherComprehensiveIncomeLossBeforeReclassificationsBeforeTax
Amounts reclassified from accumulated other comprehensive income (loss) 1,451us-gaap_ReclassificationFromAccumulatedOtherComprehensiveIncomeCurrentPeriodBeforeTax
Balance at end of period (150,046)us-gaap_AccumulatedOtherComprehensiveIncomeLossNetOfTax
Defined benefit plans adjustment | Pension  
Accumulated Other Comprehensive Income (Loss) [Line Items]  
Balance at beginning of period (36,065)us-gaap_AccumulatedOtherComprehensiveIncomeLossNetOfTax
/ us-gaap_DefinedBenefitPlansDisclosuresDefinedBenefitPlansAxis
= us-gaap_PensionPlansDefinedBenefitMember
/ us-gaap_StatementEquityComponentsAxis
= us-gaap_AccumulatedDefinedBenefitPlansAdjustmentMember
Other comprehensive income before reclassifications 203us-gaap_OtherComprehensiveIncomeLossBeforeReclassificationsBeforeTax
/ us-gaap_DefinedBenefitPlansDisclosuresDefinedBenefitPlansAxis
= us-gaap_PensionPlansDefinedBenefitMember
/ us-gaap_StatementEquityComponentsAxis
= us-gaap_AccumulatedDefinedBenefitPlansAdjustmentMember
Amounts reclassified from accumulated other comprehensive income (loss) 1,110us-gaap_ReclassificationFromAccumulatedOtherComprehensiveIncomeCurrentPeriodBeforeTax
/ us-gaap_DefinedBenefitPlansDisclosuresDefinedBenefitPlansAxis
= us-gaap_PensionPlansDefinedBenefitMember
/ us-gaap_StatementEquityComponentsAxis
= us-gaap_AccumulatedDefinedBenefitPlansAdjustmentMember
Balance at end of period (34,752)us-gaap_AccumulatedOtherComprehensiveIncomeLossNetOfTax
/ us-gaap_DefinedBenefitPlansDisclosuresDefinedBenefitPlansAxis
= us-gaap_PensionPlansDefinedBenefitMember
/ us-gaap_StatementEquityComponentsAxis
= us-gaap_AccumulatedDefinedBenefitPlansAdjustmentMember
Defined benefit plans adjustment | Postretirement Medical Benefits  
Accumulated Other Comprehensive Income (Loss) [Line Items]  
Balance at beginning of period (39,716)us-gaap_AccumulatedOtherComprehensiveIncomeLossNetOfTax
/ us-gaap_DefinedBenefitPlansDisclosuresDefinedBenefitPlansAxis
= us-gaap_OtherPostretirementBenefitPlansDefinedBenefitMember
/ us-gaap_StatementEquityComponentsAxis
= us-gaap_AccumulatedDefinedBenefitPlansAdjustmentMember
Amounts reclassified from accumulated other comprehensive income (loss) 327us-gaap_ReclassificationFromAccumulatedOtherComprehensiveIncomeCurrentPeriodBeforeTax
/ us-gaap_DefinedBenefitPlansDisclosuresDefinedBenefitPlansAxis
= us-gaap_OtherPostretirementBenefitPlansDefinedBenefitMember
/ us-gaap_StatementEquityComponentsAxis
= us-gaap_AccumulatedDefinedBenefitPlansAdjustmentMember
Balance at end of period (39,389)us-gaap_AccumulatedOtherComprehensiveIncomeLossNetOfTax
/ us-gaap_DefinedBenefitPlansDisclosuresDefinedBenefitPlansAxis
= us-gaap_OtherPostretirementBenefitPlansDefinedBenefitMember
/ us-gaap_StatementEquityComponentsAxis
= us-gaap_AccumulatedDefinedBenefitPlansAdjustmentMember
Available for sale securities  
Accumulated Other Comprehensive Income (Loss) [Line Items]  
Balance at beginning of period 413us-gaap_AccumulatedOtherComprehensiveIncomeLossNetOfTax
/ us-gaap_StatementEquityComponentsAxis
= us-gaap_AccumulatedNetUnrealizedInvestmentGainLossMember
Other comprehensive income before reclassifications 311us-gaap_OtherComprehensiveIncomeLossBeforeReclassificationsBeforeTax
/ us-gaap_StatementEquityComponentsAxis
= us-gaap_AccumulatedNetUnrealizedInvestmentGainLossMember
Amounts reclassified from accumulated other comprehensive income (loss) 14us-gaap_ReclassificationFromAccumulatedOtherComprehensiveIncomeCurrentPeriodBeforeTax
/ us-gaap_StatementEquityComponentsAxis
= us-gaap_AccumulatedNetUnrealizedInvestmentGainLossMember
Balance at end of period 738us-gaap_AccumulatedOtherComprehensiveIncomeLossNetOfTax
/ us-gaap_StatementEquityComponentsAxis
= us-gaap_AccumulatedNetUnrealizedInvestmentGainLossMember
Foreign currency translation adjustment  
Accumulated Other Comprehensive Income (Loss) [Line Items]  
Balance at beginning of period (17,880)us-gaap_AccumulatedOtherComprehensiveIncomeLossNetOfTax
/ us-gaap_StatementEquityComponentsAxis
= us-gaap_AccumulatedTranslationAdjustmentMember
Other comprehensive income before reclassifications (27,140)us-gaap_OtherComprehensiveIncomeLossBeforeReclassificationsBeforeTax
/ us-gaap_StatementEquityComponentsAxis
= us-gaap_AccumulatedTranslationAdjustmentMember
Balance at end of period (45,020)us-gaap_AccumulatedOtherComprehensiveIncomeLossNetOfTax
/ us-gaap_StatementEquityComponentsAxis
= us-gaap_AccumulatedTranslationAdjustmentMember
Tax effect of other comprehensive income gains  
Accumulated Other Comprehensive Income (Loss) [Line Items]  
Balance at beginning of period (31,048)us-gaap_AccumulatedOtherComprehensiveIncomeLossNetOfTax
/ us-gaap_StatementEquityComponentsAxis
= wlb_TaxEffectOfOtherComprehensiveIncomeGainsMember
Other comprehensive income before reclassifications (575)us-gaap_OtherComprehensiveIncomeLossBeforeReclassificationsBeforeTax
/ us-gaap_StatementEquityComponentsAxis
= wlb_TaxEffectOfOtherComprehensiveIncomeGainsMember
Balance at end of period $ (31,623)us-gaap_AccumulatedOtherComprehensiveIncomeLossNetOfTax
/ us-gaap_StatementEquityComponentsAxis
= wlb_TaxEffectOfOtherComprehensiveIncomeGainsMember