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Fair Value Disclosure (Unobservable Input Reconciliation) (Details) (USD $)
In Thousands, unless otherwise specified
12 Months Ended
Dec. 31, 2014
Dec. 31, 2013
Fair Value, Liabilities Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]    
Balance at beginning of period $ 1,581us-gaap_FairValueMeasurementWithUnobservableInputsReconciliationsRecurringBasisLiabilityValue $ 4,000us-gaap_FairValueMeasurementWithUnobservableInputsReconciliationsRecurringBasisLiabilityValue
Change in estimate (1,614)nacco_FairValueMeasurementwithUnobservableInputsReconciliationRecurringBasisLiabilityEstimateIncreaseDecrease (2,400)nacco_FairValueMeasurementwithUnobservableInputsReconciliationRecurringBasisLiabilityEstimateIncreaseDecrease
Accretion expense 33us-gaap_FairValueMeasurementWithUnobservableInputsReconciliationRecurringBasisLiabilityGainLossIncludedInEarnings  
Balance at end of period $ 0us-gaap_FairValueMeasurementWithUnobservableInputsReconciliationsRecurringBasisLiabilityValue $ 1,581us-gaap_FairValueMeasurementWithUnobservableInputsReconciliationsRecurringBasisLiabilityValue