Fair value measurements - Fair Value Measurements on a Recurring Basis (Details) $ in Thousands	12 Months Ended
Dec. 31, 2018 USD ($)	Dec. 31, 2017 USD ($)
Fair value measurements on a recurring and nonrecurring basis
Available-for-sale, at fair value	$ 1,388,533	$ 1,401,198
Discount rate
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Mortgage revenue bonds, measurement input	0.0396
Mortgage-backed securities — issued or guaranteed by U.S. Government agencies or sponsored agencies
Fair value measurements on a recurring and nonrecurring basis
Available-for-sale, at fair value	$ 1,161,416	1,201,473
U.S. Treasury and federal agency obligations
Fair value measurements on a recurring and nonrecurring basis
Available-for-sale, at fair value	154,349	184,298
Mortgage revenue bonds
Fair value measurements on a recurring and nonrecurring basis
Available-for-sale, at fair value	23,636	15,427
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Balance, January 1	15,427	15,427
Principal payments received	0	0
Purchases	8,209	0
Unrealized gain (loss) included in other comprehensive income	0	0
Balance, December 31	23,636	15,427
Fair value measurements on a recurring basis \| Level 1
Fair value measurements on a recurring and nonrecurring basis
Derivative assets	0	0
Derivative liabilities	34	20
Fair value measurements on a recurring basis \| Level 1 \| Bank
Fair value measurements on a recurring and nonrecurring basis
Available-for-sale, at fair value	0	0
Fair value measurements on a recurring basis \| Level 1 \| Bank \| Interest Rate Lock Commitments
Fair value measurements on a recurring and nonrecurring basis
Derivative assets	0	0
Derivative liabilities	0	0
Fair value measurements on a recurring basis \| Level 1 \| Bank \| Forward commitments
Fair value measurements on a recurring and nonrecurring basis
Derivative assets	0	0
Derivative liabilities	34	20
Fair value measurements on a recurring basis \| Level 1 \| Bank \| Mortgage-backed securities — issued or guaranteed by U.S. Government agencies or sponsored agencies
Fair value measurements on a recurring and nonrecurring basis
Available-for-sale, at fair value	0	0
Fair value measurements on a recurring basis \| Level 1 \| Bank \| U.S. Treasury and federal agency obligations
Fair value measurements on a recurring and nonrecurring basis
Available-for-sale, at fair value	0	0
Fair value measurements on a recurring basis \| Level 1 \| Bank \| Corporate bonds
Fair value measurements on a recurring and nonrecurring basis
Available-for-sale, at fair value	0	0
Fair value measurements on a recurring basis \| Level 1 \| Bank \| Mortgage revenue bonds
Fair value measurements on a recurring and nonrecurring basis
Available-for-sale, at fair value	0	0
Fair value measurements on a recurring basis \| Level 1 \| Electric utility \| Window forward contracts
Fair value measurements on a recurring and nonrecurring basis
Derivative assets	0	0
Fair value measurements on a recurring basis \| Level 1 \| Other \| Interest rate swap
Fair value measurements on a recurring and nonrecurring basis
Derivative liabilities	0	0
Fair value measurements on a recurring basis \| Level 2
Fair value measurements on a recurring and nonrecurring basis
Derivative assets	91	393
Derivative liabilities	596	10
Fair value measurements on a recurring basis \| Level 2 \| Bank
Fair value measurements on a recurring and nonrecurring basis
Available-for-sale, at fair value	1,364,897	1,385,771
Fair value measurements on a recurring basis \| Level 2 \| Bank \| Interest Rate Lock Commitments
Fair value measurements on a recurring and nonrecurring basis
Derivative assets	91	133
Derivative liabilities	0	2
Fair value measurements on a recurring basis \| Level 2 \| Bank \| Forward commitments
Fair value measurements on a recurring and nonrecurring basis
Derivative assets	0	4
Derivative liabilities	9	8
Fair value measurements on a recurring basis \| Level 2 \| Bank \| Mortgage-backed securities — issued or guaranteed by U.S. Government agencies or sponsored agencies
Fair value measurements on a recurring and nonrecurring basis
Available-for-sale, at fair value	1,161,416	1,201,473
Fair value measurements on a recurring basis \| Level 2 \| Bank \| U.S. Treasury and federal agency obligations
Fair value measurements on a recurring and nonrecurring basis
Available-for-sale, at fair value	154,349	184,298
Fair value measurements on a recurring basis \| Level 2 \| Bank \| Corporate bonds
Fair value measurements on a recurring and nonrecurring basis
Available-for-sale, at fair value	49,132	0
Fair value measurements on a recurring basis \| Level 2 \| Bank \| Mortgage revenue bonds
Fair value measurements on a recurring and nonrecurring basis
Available-for-sale, at fair value	0	0
Fair value measurements on a recurring basis \| Level 2 \| Electric utility \| Window forward contracts
Fair value measurements on a recurring and nonrecurring basis
Derivative assets	0	256
Fair value measurements on a recurring basis \| Level 2 \| Other \| Interest rate swap
Fair value measurements on a recurring and nonrecurring basis
Derivative liabilities	587	0
Fair value measurements on a recurring basis \| Level 3
Fair value measurements on a recurring and nonrecurring basis
Derivative assets	0	0
Derivative liabilities	0	0
Fair value measurements on a recurring basis \| Level 3 \| Bank
Fair value measurements on a recurring and nonrecurring basis
Available-for-sale, at fair value	23,636	15,427
Fair value measurements on a recurring basis \| Level 3 \| Bank \| Interest Rate Lock Commitments
Fair value measurements on a recurring and nonrecurring basis
Derivative assets	0	0
Derivative liabilities	0	0
Fair value measurements on a recurring basis \| Level 3 \| Bank \| Forward commitments
Fair value measurements on a recurring and nonrecurring basis
Derivative assets	0	0
Derivative liabilities	0	0
Fair value measurements on a recurring basis \| Level 3 \| Bank \| Mortgage-backed securities — issued or guaranteed by U.S. Government agencies or sponsored agencies
Fair value measurements on a recurring and nonrecurring basis
Available-for-sale, at fair value	0	0
Fair value measurements on a recurring basis \| Level 3 \| Bank \| U.S. Treasury and federal agency obligations
Fair value measurements on a recurring and nonrecurring basis
Available-for-sale, at fair value	0	0
Fair value measurements on a recurring basis \| Level 3 \| Bank \| Corporate bonds
Fair value measurements on a recurring and nonrecurring basis
Available-for-sale, at fair value	0	0
Fair value measurements on a recurring basis \| Level 3 \| Bank \| Mortgage revenue bonds
Fair value measurements on a recurring and nonrecurring basis
Available-for-sale, at fair value	23,636	15,427
Fair value measurements on a recurring basis \| Level 3 \| Electric utility \| Window forward contracts
Fair value measurements on a recurring and nonrecurring basis
Derivative assets	0	0
Fair value measurements on a recurring basis \| Level 3 \| Other \| Interest rate swap
Fair value measurements on a recurring and nonrecurring basis
Derivative liabilities	$ 0	$ 0

Amount of investment in debt security measured at fair value with change in fair value recognized in other comprehensive income (available-for-sale).

Reference 1: http://www.xbrl.org/2003/role/disclosureRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 320
-SubTopic 10
-Section 45
-Paragraph 1
-Subparagraph (b)
-URI http://asc.fasb.org/extlink&oid=109979856&loc=d3e26610-111562

Reference 2: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 320
-SubTopic 10
-Section 50
-Paragraph 2
-Subparagraph (aa)
-URI http://asc.fasb.org/extlink&oid=82897076&loc=d3e27161-111563

+ Details

Name:	us-gaap_AvailableForSaleSecuritiesDebtSecurities
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	instant

- Definition

Value of input used to measure investment in debt security measured at fair value with change in fair value recognized in other comprehensive income (available-for-sale).

+ References

Reference 1: http://www.xbrl.org/2003/role/disclosureRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 820
-SubTopic 10
-Section 50
-Paragraph 2
-Subparagraph (bbb)(2)
-URI http://asc.fasb.org/extlink&oid=99377789&loc=d3e19207-110258

+ Details

Name:	us-gaap_DebtSecuritiesAvailableForSaleMeasurementInput
Namespace Prefix:	us-gaap_
Data Type:	xbrli:decimalItemType
Balance Type:	na
Period Type:	instant

- Definition

Fair value, after the effects of master netting arrangements, of a financial asset or other contract with one or more underlyings, notional amount or payment provision or both, and the contract can be net settled by means outside the contract or delivery of an asset. Includes assets not subject to a master netting arrangement and not elected to be offset.

+ References

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 210
-SubTopic 20
-Section 55
-Paragraph 10
-URI http://asc.fasb.org/extlink&oid=99393222&loc=SL20226008-175313

Reference 2: http://www.xbrl.org/2003/role/disclosureRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 825
-SubTopic 10
-Section 50
-Paragraph 10
-URI http://asc.fasb.org/extlink&oid=109250915&loc=d3e13433-108611

Reference 3: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 210
-SubTopic 20
-Section 50
-Paragraph 3
-Subparagraph (c)
-URI http://asc.fasb.org/extlink&oid=51824906&loc=SL20225862-175312

+ Details

Name:	us-gaap_DerivativeAssets
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	instant

- Definition

Fair value, after the effects of master netting arrangements, of a financial liability or contract with one or more underlyings, notional amount or payment provision or both, and the contract can be net settled by means outside the contract or delivery of an asset. Includes liabilities not subject to a master netting arrangement and not elected to be offset.

+ References

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 210
-SubTopic 20
-Section 50
-Paragraph 3
-Subparagraph (c)
-URI http://asc.fasb.org/extlink&oid=51824906&loc=SL20225862-175312

Reference 2: http://www.xbrl.org/2003/role/disclosureRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 825
-SubTopic 10
-Section 50
-Paragraph 10
-URI http://asc.fasb.org/extlink&oid=109250915&loc=d3e13433-108611

Reference 3: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 210
-SubTopic 20
-Section 55
-Paragraph 10
-URI http://asc.fasb.org/extlink&oid=99393222&loc=SL20226008-175313

+ Details

Name:	us-gaap_DerivativeLiabilities
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	instant

- Definition

Line items represent financial concepts included in a table. These concepts are used to disclose reportable information associated with domain members defined in one or many axes to the table.

+ References

No definition available.

+ Details

Name:	us-gaap_FairValueAssetsAndLiabilitiesMeasuredOnRecurringAndNonrecurringBasisLineItems
Namespace Prefix:	us-gaap_
Data Type:	xbrli:stringItemType
Balance Type:	na
Period Type:	duration

- Definition

A roll forward is a reconciliation of a concept from the beginning of a period to the end of a period.

+ References

No definition available.

+ Details

Name:	us-gaap_FairValueAssetsMeasuredOnRecurringBasisUnobservableInputReconciliationCalculationRollForward
Namespace Prefix:	us-gaap_
Data Type:	xbrli:stringItemType
Balance Type:	na
Period Type:	duration

- Definition

Amount of gain (loss) recognized in other comprehensive income (loss) for financial instrument classified as an asset measured using unobservable inputs that reflect the entity's own assumption about the assumptions market participants would use in pricing.

+ References

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 820
-SubTopic 10
-Section 50
-Paragraph 2
-Subparagraph (c)(1a)
-URI http://asc.fasb.org/extlink&oid=99377789&loc=d3e19207-110258

+ Details

Name:	us-gaap_FairValueMeasurementWithUnobservableInputsReconciliationRecurringBasisAssetGainLossIncludedInOtherComprehensiveIncomeLoss
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	duration

- Definition

Amount of purchases of financial instrument classified as an asset measured using unobservable inputs that reflect the entity's own assumption about the assumptions market participants would use in pricing.

+ References

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 820
-SubTopic 10
-Section 50
-Paragraph 2
-Subparagraph (c)(2)
-URI http://asc.fasb.org/extlink&oid=99377789&loc=d3e19207-110258

+ Details

Name:	us-gaap_FairValueMeasurementWithUnobservableInputsReconciliationRecurringBasisAssetPurchases
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	duration

- Definition

Amount of settlement of financial instrument classified as an asset measured using unobservable inputs that reflect the entity's own assumption about the assumptions market participants would use in pricing.

+ References

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 820
-SubTopic 10
-Section 50
-Paragraph 2
-Subparagraph (c)(2)
-URI http://asc.fasb.org/extlink&oid=99377789&loc=d3e19207-110258

+ Details

Name:	us-gaap_FairValueMeasurementWithUnobservableInputsReconciliationRecurringBasisAssetSettlements
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	duration

- Definition

Fair value of financial instrument classified as an asset measured using unobservable inputs that reflect the entity's own assumption about the assumptions market participants would use in pricing.

+ References

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 820
-SubTopic 10
-Section 50
-Paragraph 2
-Subparagraph (c)
-URI http://asc.fasb.org/extlink&oid=99377789&loc=d3e19207-110258

+ Details

Name:	us-gaap_FairValueMeasurementWithUnobservableInputsReconciliationRecurringBasisAssetValue
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	instant

- Details

Name:	us-gaap_MeasurementInputTypeAxis=us-gaap_MeasurementInputDiscountRateMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_FinancialInstrumentAxis=us-gaap_MortgageBackedSecuritiesIssuedByUSGovernmentSponsoredEnterprisesMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_FinancialInstrumentAxis=us-gaap_USTreasuryAndGovernmentMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_FinancialInstrumentAxis=us-gaap_AssetBackedSecuritiesMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_FairValueByMeasurementFrequencyAxis=us-gaap_FairValueMeasurementsRecurringMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_FairValueByFairValueHierarchyLevelAxis=us-gaap_FairValueInputsLevel1Member
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_StatementBusinessSegmentsAxis=he_BankingSegmentMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_DerivativeInstrumentRiskAxis=us-gaap_InterestRateLockCommitmentsMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_DerivativeInstrumentRiskAxis=us-gaap_ForwardContractsMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_FinancialInstrumentAxis=us-gaap_CorporateBondSecuritiesMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_StatementBusinessSegmentsAxis=he_ElectricUtilitySegmentMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_DerivativeInstrumentRiskAxis=us-gaap_ForeignExchangeForwardMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_StatementBusinessSegmentsAxis=us-gaap_AllOtherSegmentsMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_DerivativeInstrumentRiskAxis=us-gaap_InterestRateSwapMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_FairValueByFairValueHierarchyLevelAxis=us-gaap_FairValueInputsLevel2Member
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_FairValueByFairValueHierarchyLevelAxis=us-gaap_FairValueInputsLevel3Member
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type: