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Pension Plans and Other Postretirement Benefits (Details 6) (USD $)
In Thousands, unless otherwise specified
 12 Months Ended
Dec. 31, 2014
Dec. 31, 2013
Dec. 31, 2012
Accumulated postretirement benefit obligations and Service and interest cost components of the net periodic postretirement benefit expense      
Accumulated postretirement benefit obligations Effect of a one percentage point increase    [1] $ 0us-gaap_DefinedBenefitPlanEffectOfOnePercentagePointIncreaseOnAccumulatedPostretirementBenefitObligation $ 45us-gaap_DefinedBenefitPlanEffectOfOnePercentagePointIncreaseOnAccumulatedPostretirementBenefitObligation
Accumulated postretirement benefit obligations Effect of a one percentage point decrease    [1] 0us-gaap_DefinedBenefitPlanEffectOfOnePercentagePointDecreaseOnAccumulatedPostretirementBenefitObligation1 (43)us-gaap_DefinedBenefitPlanEffectOfOnePercentagePointDecreaseOnAccumulatedPostretirementBenefitObligation1
Service and interest cost components of the net periodic postretirement benefit expense Effect of a one percentage point increase    [1] 2us-gaap_DefinedBenefitPlanEffectOfOnePercentagePointIncreaseOnServiceAndInterestCostComponents 1us-gaap_DefinedBenefitPlanEffectOfOnePercentagePointIncreaseOnServiceAndInterestCostComponents
Service and interest cost components of the net periodic postretirement benefit expense Effect of a one percentage point decrease    [1] $ (2)us-gaap_DefinedBenefitPlanEffectOfOnePercentagePointDecreaseOnServiceAndInterestCostComponents1 $ (1)us-gaap_DefinedBenefitPlanEffectOfOnePercentagePointDecreaseOnServiceAndInterestCostComponents1
Weighted-average assumptions used to determine benefit obligations as of December 31:      
Discount rate 3.66%us-gaap_DefinedBenefitPlanAssumptionsUsedCalculatingBenefitObligationDiscountRate 4.46%us-gaap_DefinedBenefitPlanAssumptionsUsedCalculatingBenefitObligationDiscountRate 3.51%us-gaap_DefinedBenefitPlanAssumptionsUsedCalculatingBenefitObligationDiscountRate
Healthcare cost trend rate    [1]    [1] 7.50%hmn_DefinedBenefitPlanHealthCareBenefitObligationsTrendRateAssumedForNextFiscalYear
Rate to which the cost trend rate is assumed to decline (ultimate trend rate)    [1]    [1] 5.00%hmn_DefinedBenefitPlanUltimateHealthCareBenefitObligationsTrendRate
Year the rate is assumed to reach the ultimate trend rate     2022
Expected return on plan assets    [1]    [1]    [1]
Weighted-average assumptions used to determine net periodic benefit cost for the years ended December 31:      
Discount rate 4.46%us-gaap_DefinedBenefitPlanAssumptionsUsedCalculatingNetPeriodicBenefitCostDiscountRate 3.51%us-gaap_DefinedBenefitPlanAssumptionsUsedCalculatingNetPeriodicBenefitCostDiscountRate 2.95%us-gaap_DefinedBenefitPlanAssumptionsUsedCalculatingNetPeriodicBenefitCostDiscountRate
Healthcare cost trend rate    [1] 7.50%us-gaap_DefinedBenefitPlanHealthCareCostTrendRateAssumedForNextFiscalYear 8.00%us-gaap_DefinedBenefitPlanHealthCareCostTrendRateAssumedForNextFiscalYear
Rate to which the cost trend rate is assumed to decline (ultimate trend rate)    [1] 5.00%us-gaap_DefinedBenefitPlanUltimateHealthCareCostTrendRate 5.00%us-gaap_DefinedBenefitPlanUltimateHealthCareCostTrendRate
Year the rate is assumed to reach the ultimate trend rate   2022 2022
Expected return on plan assets    [1]    [1]    [1]
[1] Not applicable.