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Shareholders' equity - Additional Information (Details) - USD ($) $ / shares in Units, $ in Millions	Dec. 31, 2017	Mar. 20, 2015	Mar. 19, 2013
Equity [Abstract]
Common stock reserved for future issuance (in shares)	12,158,460
Public offering related to equity forward transactions (in shares)			6,100,000
Sale of common stock related to equity forward transaction (in dollars per share)			$ 26.75
Delivery of net shares on settlement (in shares)		4,700,000
Delivery of cash on settlement		$ 104.5
Underwriting discount (in dollars per share)		$ 4.7

X

- Definition

Represents the amount of delivery of cash to forward counterparty on settlement of equity forward transaction.

+ References

No definition available.

+ Details

Name:	he_EquityForwardTransactionDeliveryOfCashOnSettlement
Namespace Prefix:	he_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	instant

X

- Definition

Represents the amount of delivery of net shares to forward counterparty on settlement of equity forward transaction.

+ References

No definition available.

+ Details

Name:	he_EquityForwardTransactionDeliveryOfNetSharesOnSettlement
Namespace Prefix:	he_
Data Type:	xbrli:sharesItemType
Balance Type:	na
Period Type:	instant

X

- Definition

Represents the per share amount for each share of common stock sold to group of underwriters.

+ References

No definition available.

+ Details

Name:	he_EquityForwardTransactionSaleOfStockPricePerShare
Namespace Prefix:	he_
Data Type:	num:perShareItemType
Balance Type:	na
Period Type:	instant

X

- Definition

Represents the number of shares in connection with public offering under the equity forward transaction.

+ References

No definition available.

+ Details

Name:	he_EquityForwardTransactionSharesInConnectionWithPublicOffering
Namespace Prefix:	he_
Data Type:	xbrli:sharesItemType
Balance Type:	na
Period Type:	instant

X

- Definition

Represents the underwriting discount on sale of common stock to underwriters.

+ References

No definition available.

+ Details

Name:	he_EquityForwardTransactionUnderwritingDiscount
Namespace Prefix:	he_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	instant

X

- Definition

Aggregate number of common shares reserved for future issuance.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 210
-SubTopic 10
-Section S99
-Paragraph 1
-Subparagraph (SX 210.5-02.29)
-URI http://asc.fasb.org/extlink&oid=6877327&loc=d3e13212-122682

+ Details

Name:	us-gaap_CommonStockCapitalSharesReservedForFutureIssuance
Namespace Prefix:	us-gaap_
Data Type:	xbrli:sharesItemType
Balance Type:	na
Period Type:	instant

X

- References

No definition available.

+ Details

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