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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
 12 Months Ended
Dec. 31, 2014
Dec. 31, 2013
Dec. 31, 2012
Heritage Health Benefit Expenses [Abstract]      
Health care benefits $ 9,473wlb_HealthCareBenefits $ 12,579wlb_HealthCareBenefits $ 11,367wlb_HealthCareBenefits
Combined benefit fund payments 1,966wlb_CombinedBenefitFundPayments 2,240wlb_CombinedBenefitFundPayments 2,258wlb_CombinedBenefitFundPayments
Workers' compensation benefits (credit) 230wlb_WorkersCompensationBenefits (1,212)wlb_WorkersCompensationBenefits (1,322)wlb_WorkersCompensationBenefits
Black lung benefits (credit) 1,719wlb_BlackLungBenefits (189)wlb_BlackLungBenefits 1,085wlb_BlackLungBenefits
Total $ 13,388wlb_HealthBenefitExpenses $ 13,418wlb_HealthBenefitExpenses $ 13,388wlb_HealthBenefitExpenses