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Shareholders' equity - Additional Information (Details) - USD ($) $ / shares in Units, $ in Millions | Dec. 31, 2017 | Mar. 20, 2015 | Mar. 19, 2013
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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 |

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- Definition Represents the amount of delivery of cash to forward counterparty on settlement of equity forward transaction. + References No definition available. + Details
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Data Type: | xbrli:monetaryItemType
Balance Type: | credit
Period Type: | instant

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- 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

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- Definition Represents the per share amount for each share of common stock sold to group of underwriters. + References No definition available. + Details
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Namespace Prefix: | he_
Data Type: | num:perShareItemType
Balance Type: | na
Period Type: | instant

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- Definition Represents the number of shares in connection with public offering under the equity forward transaction. + References No definition available. + Details
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Namespace Prefix: | he_
Data Type: | xbrli:sharesItemType
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- 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

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- 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
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- References No definition available. + Details
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