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HERITAGE HEALTH BENEFIT EXPENSES - Components of Costs of Benefits to Former Mining Operation Employees (Detail) (USD $)
In Thousands, unless otherwise specified
 3 Months Ended
Mar. 31, 2015
Mar. 31, 2014
Heritage Health Benefit Expenses [Abstract]    
Health care benefits $ 2,049wlb_HealthCareBenefits $ 2,276wlb_HealthCareBenefits
Combined benefit fund payments 462wlb_CombinedBenefitFundPayments 512wlb_CombinedBenefitFundPayments
Workers' compensation benefits 110wlb_WorkersCompensationBenefits 136wlb_WorkersCompensationBenefits
Black lung benefits 438wlb_BlackLungBenefits 620wlb_BlackLungBenefits
Total $ 3,059wlb_HealthBenefitExpenses $ 3,544wlb_HealthBenefitExpenses