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Share-based compensation Additional Information (Details) (USD $)
In Millions, except Share data, unless otherwise specified
12 Months Ended 0 Months Ended
Dec. 31, 2014
Dec. 31, 2013
Dec. 31, 2012
Mar. 01, 2014
Share-based compensation        
Income tax benefit (in dollars) $ 3.4us-gaap_EmployeeServiceShareBasedCompensationTaxBenefitFromCompensationExpense $ 2.8us-gaap_EmployeeServiceShareBasedCompensationTaxBenefitFromCompensationExpense $ 2.4us-gaap_EmployeeServiceShareBasedCompensationTaxBenefitFromCompensationExpense  
Equity and Incentive Plan (EIP)        
Share-based compensation        
Additional shares available for issuance       1,500,000us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardNumberOfAdditionalSharesAuthorized
/ us-gaap_PlanNameAxis
= he_EquityAndIncentivePlanMember
Shares remaining available for future issuance 3,600,000he_ShareBasedCompensationNumberOfSharesThatCouldBeIssuedUponVestingOfRestrictedStockUnitsAndLongTermIncentivePlans
/ us-gaap_PlanNameAxis
= he_EquityAndIncentivePlanMember
     
Shares that can be issued upon vesting of outstanding units and achievement of performance goals 900,000he_ShareBasedCompensationNumberOfSharesThatCouldBeIssuedUponVestingAndAchievementOfPerformanceGoals
/ us-gaap_PlanNameAxis
= he_EquityAndIncentivePlanMember
     
Stock Option and Incentive Plan (SOIP)        
Share-based compensation        
Outstanding shares 17,000he_ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsAndOtherAwardsOutstandingNumber
/ us-gaap_PlanNameAxis
= he_StockOptionAndIncentivePlanMember
     
Period of time from grant date after which awards become unrestricted 4 years      
Nonemployee Director Stock Plan        
Share-based compensation        
Shares remaining available for future issuance 169,290us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardNumberOfSharesAvailableForGrant
/ us-gaap_PlanNameAxis
= he_NonemployeeDirectorStockPlanMember
     
Long-term incentive plan (LTIP)        
Share-based compensation        
Performance period 3 years      
Exception to forfeiture, minimum requisite service period 12 months      
Payout low end of range (as a percent) 0.00%he_ShareBasedCompensationByShareBasedPaymentAwardPayoutLowOfRange
/ us-gaap_AwardTypeAxis
= he_LongTermIncentivePlanMember
     
Payout high end of range (as a percent) 200.00%he_ShareBasedCompensationByShareBasedPaymentAwardPayoutHighEndOfRange
/ us-gaap_AwardTypeAxis
= he_LongTermIncentivePlanMember
     
Stock appreciation rights (SARs)        
Share-based compensation        
Intrinsic value of equity instruments other than options, vested 0.6he_SharebasedCompensationArrangementbySharebasedPaymentAwardEquityInstrumentsOtherthanOptionsAggregateIntrinsicValueVested
/ us-gaap_AwardTypeAxis
= us-gaap_StockAppreciationRightsSARSMember
     
Expired (in shares) 0he_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsExpirationsInPeriod
/ us-gaap_AwardTypeAxis
= us-gaap_StockAppreciationRightsSARSMember
0he_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsExpirationsInPeriod
/ us-gaap_AwardTypeAxis
= us-gaap_StockAppreciationRightsSARSMember
4,000he_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsExpirationsInPeriod
/ us-gaap_AwardTypeAxis
= us-gaap_StockAppreciationRightsSARSMember
 
Stock appreciation rights (SARs) | Stock Option and Incentive Plan (SOIP)        
Share-based compensation        
Percentage of award exercisable in installments 25.00%he_SharebasedCompensationArrangementbySharebasedPaymentAwardAwardVestingRightsPercentage
/ us-gaap_AwardTypeAxis
= us-gaap_StockAppreciationRightsSARSMember
/ us-gaap_PlanNameAxis
= he_StockOptionAndIncentivePlanMember
     
Period during which the award vests and becomes exercisable in installments 4 years      
Period award expires if not exercised 10 years      
Restricted shares        
Share-based compensation        
Fair value of vested stock (in dollars) 0.1us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsVestedInPeriodTotalFairValue
/ us-gaap_AwardTypeAxis
= us-gaap_RestrictedStockMember
0.1us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsVestedInPeriodTotalFairValue
/ us-gaap_AwardTypeAxis
= us-gaap_RestrictedStockMember
0.9us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsVestedInPeriodTotalFairValue
/ us-gaap_AwardTypeAxis
= us-gaap_RestrictedStockMember
 
Income tax benefit (in dollars)     0.2us-gaap_EmployeeServiceShareBasedCompensationTaxBenefitFromCompensationExpense
/ us-gaap_AwardTypeAxis
= us-gaap_RestrictedStockMember
 
Shares vested 4,503us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsVestedInPeriod
/ us-gaap_AwardTypeAxis
= us-gaap_RestrictedStockMember
4,502us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsVestedInPeriod
/ us-gaap_AwardTypeAxis
= us-gaap_RestrictedStockMember
37,802us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsVestedInPeriod
/ us-gaap_AwardTypeAxis
= us-gaap_RestrictedStockMember
 
Restricted shares | Equity and Incentive Plan (EIP)        
Share-based compensation        
The number of equal annual increments for which the awards become unrestricted (in installments) 4he_ShareBasedCompensationArrangementByShareBasedPaymentAwardAnnualIncrementsUponWhichRestrictedAwardsBecomeUnrestrictedNumber
/ us-gaap_AwardTypeAxis
= us-gaap_RestrictedStockMember
/ us-gaap_PlanNameAxis
= he_EquityAndIncentivePlanMember
     
Restricted stock units        
Share-based compensation        
Fair value of vested stock (in dollars) 4.1us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsVestedInPeriodTotalFairValue
/ us-gaap_AwardTypeAxis
= us-gaap_RestrictedStockUnitsRSUMember
3.7us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsVestedInPeriodTotalFairValue
/ us-gaap_AwardTypeAxis
= us-gaap_RestrictedStockUnitsRSUMember
0.7us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsVestedInPeriodTotalFairValue
/ us-gaap_AwardTypeAxis
= us-gaap_RestrictedStockUnitsRSUMember
 
Income tax benefit (in dollars) 1.2us-gaap_EmployeeServiceShareBasedCompensationTaxBenefitFromCompensationExpense
/ us-gaap_AwardTypeAxis
= us-gaap_RestrictedStockUnitsRSUMember
0.9us-gaap_EmployeeServiceShareBasedCompensationTaxBenefitFromCompensationExpense
/ us-gaap_AwardTypeAxis
= us-gaap_RestrictedStockUnitsRSUMember
0.2us-gaap_EmployeeServiceShareBasedCompensationTaxBenefitFromCompensationExpense
/ us-gaap_AwardTypeAxis
= us-gaap_RestrictedStockUnitsRSUMember
 
Unrecognized compensation cost 4.4us-gaap_EmployeeServiceShareBasedCompensationNonvestedAwardsTotalCompensationCostNotYetRecognized
/ us-gaap_AwardTypeAxis
= us-gaap_RestrictedStockUnitsRSUMember
     
Weighted-average period over which unrecognized compensation cost expected to be recognized 2 years 5 months 24 days      
Shares vested 144,702us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsVestedInPeriod
/ us-gaap_AwardTypeAxis
= us-gaap_RestrictedStockUnitsRSUMember
118,885us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsVestedInPeriod
/ us-gaap_AwardTypeAxis
= us-gaap_RestrictedStockUnitsRSUMember
25,728us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsVestedInPeriod
/ us-gaap_AwardTypeAxis
= us-gaap_RestrictedStockUnitsRSUMember
 
LTIP linked to TRS        
Share-based compensation        
Fair value of vested stock (in dollars) 0us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsVestedInPeriodTotalFairValue
/ us-gaap_AwardTypeAxis
= he_LongTermIncentivePlanLinkedToTotalReturnToShareholdersMember
2.2us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsVestedInPeriodTotalFairValue
/ us-gaap_AwardTypeAxis
= he_LongTermIncentivePlanLinkedToTotalReturnToShareholdersMember
0.6us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsVestedInPeriodTotalFairValue
/ us-gaap_AwardTypeAxis
= he_LongTermIncentivePlanLinkedToTotalReturnToShareholdersMember
 
Income tax benefit (in dollars) 0us-gaap_EmployeeServiceShareBasedCompensationTaxBenefitFromCompensationExpense
/ us-gaap_AwardTypeAxis
= he_LongTermIncentivePlanLinkedToTotalReturnToShareholdersMember
0.9us-gaap_EmployeeServiceShareBasedCompensationTaxBenefitFromCompensationExpense
/ us-gaap_AwardTypeAxis
= he_LongTermIncentivePlanLinkedToTotalReturnToShareholdersMember
0.2us-gaap_EmployeeServiceShareBasedCompensationTaxBenefitFromCompensationExpense
/ us-gaap_AwardTypeAxis
= he_LongTermIncentivePlanLinkedToTotalReturnToShareholdersMember
 
Unrecognized compensation cost 2.2us-gaap_EmployeeServiceShareBasedCompensationNonvestedAwardsTotalCompensationCostNotYetRecognized
/ us-gaap_AwardTypeAxis
= he_LongTermIncentivePlanLinkedToTotalReturnToShareholdersMember
     
Weighted-average period over which unrecognized compensation cost expected to be recognized 1 year 6 months      
Shares vested 70,189us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsVestedInPeriod
/ us-gaap_AwardTypeAxis
= he_LongTermIncentivePlanLinkedToTotalReturnToShareholdersMember
87,753us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsVestedInPeriod
/ us-gaap_AwardTypeAxis
= he_LongTermIncentivePlanLinkedToTotalReturnToShareholdersMember
35,397us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsVestedInPeriod
/ us-gaap_AwardTypeAxis
= he_LongTermIncentivePlanLinkedToTotalReturnToShareholdersMember
 
Shares settled 70,205he_SharebasedCompensationArrangementbySharebasedPaymentAwardEquityInstrumentsOtherThanOptionsSettled
/ us-gaap_AwardTypeAxis
= he_LongTermIncentivePlanLinkedToTotalReturnToShareholdersMember
     
Expired (in shares) 17,548he_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsExpirationsInPeriod
/ us-gaap_AwardTypeAxis
= he_LongTermIncentivePlanLinkedToTotalReturnToShareholdersMember
     
LTIP awards linked to other performance conditions        
Share-based compensation        
Fair value of vested stock (in dollars) 1.9us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsVestedInPeriodTotalFairValue
/ us-gaap_AwardTypeAxis
= he_LongTermIncentivePlanAwardsLinkedToOtherPerformanceConditionsMember
0.6us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsVestedInPeriodTotalFairValue
/ us-gaap_AwardTypeAxis
= he_LongTermIncentivePlanAwardsLinkedToOtherPerformanceConditionsMember
   
Income tax benefit (in dollars) 0.8us-gaap_EmployeeServiceShareBasedCompensationTaxBenefitFromCompensationExpense
/ us-gaap_AwardTypeAxis
= he_LongTermIncentivePlanAwardsLinkedToOtherPerformanceConditionsMember
0.2us-gaap_EmployeeServiceShareBasedCompensationTaxBenefitFromCompensationExpense
/ us-gaap_AwardTypeAxis
= he_LongTermIncentivePlanAwardsLinkedToOtherPerformanceConditionsMember
   
Shares vested 65,089us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsVestedInPeriod
/ us-gaap_AwardTypeAxis
= he_LongTermIncentivePlanAwardsLinkedToOtherPerformanceConditionsMember
18,280us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsVestedInPeriod
/ us-gaap_AwardTypeAxis
= he_LongTermIncentivePlanAwardsLinkedToOtherPerformanceConditionsMember
0us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsVestedInPeriod
/ us-gaap_AwardTypeAxis
= he_LongTermIncentivePlanAwardsLinkedToOtherPerformanceConditionsMember
 
2012-2014 LTIP linked to other performance conditions        
Share-based compensation        
Unrecognized compensation cost $ 3.4us-gaap_EmployeeServiceShareBasedCompensationNonvestedAwardsTotalCompensationCostNotYetRecognized
/ us-gaap_AwardTypeAxis
= he_LongTermIncentive2012To2014PlanLinkedToOtherPerformanceConditionsMember
     
Weighted-average period over which unrecognized compensation cost expected to be recognized 1 year 6 months