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Shareholders' equity Additional Information (Details) (USD $) In Millions, except Share data, unless otherwise specified | Dec. 31, 2014 | Jul. 14, 2014 | Dec. 19, 2013 | Mar. 20, 2013 | Mar. 19, 2013
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Equity [Abstract] |  |  |  |  |
Common stock reserved for future issuance | 18,372,187 us-gaap_CommonStockCapitalSharesReservedForFutureIssuance |  |  |  |
Common stock reserved for future issuance in equity forward transaction | 4,700,000 he_CommonStockCapitalSharesReservedforFutureIssuanceEquityForwardTransaction |  |  |  |
Public offering related to equity forward transaction (in shares) |  |  |  |  | 6,100,000 he_EquityForwardTransactionSharesInConnectionWithPublicOffering
Sale of common stock related to equity forward transaction (in dollars per share) |  |  |  |  | $ 26.75 he_EquityForwardTransactionSaleOfStockPricePerShare
Closing price of common stock (in dollar per share) |  |  |  |  | $ 27.01 us-gaap_SharePrice
Offering related to underwriters exercising their over-allotment option under equity forward transaction (in shares) |  |  |  | 900,000 he_EquityForwardTransactionSharesInConnectionWithUnderwritersExercisingOverAllotmentOption |
Shares borrowed by forward counterparty from third party |  |  |  |  | 7,000,000 he_EquityForwardTransactionSharesBorrowed
Underwriting discount (in dollars per share) |  |  |  |  | $ 1.00312 he_EquityForwardTransactionUnderwritingDiscount
Forward sale price (in dollars per share) | $ 22.63 he_EquityForwardTransactionForwardSalePrice |  |  |  | $ 25.74688 he_EquityForwardTransactionForwardSalePrice
Initial fair value (in dollars per share) | $ 0 he_EquityForwardTransactionInitialFairValue |  |  |  |
Delivery of net shares on settlement | 1,500,000 he_EquityForwardTransactionDeliveryOfNetSharesOnSettlement | 1,000,000 he_EquityForwardTransactionDeliveryOfNetSharesOnSettlement | 1,300,000 he_EquityForwardTransactionDeliveryOfNetSharesOnSettlement |  |
Delivery of cash on settlement | $ 51.0 he_EquityForwardTransactionDeliveryOfCashOnSettlement | $ 23.9 he_EquityForwardTransactionDeliveryOfCashOnSettlement | $ 32.1 he_EquityForwardTransactionDeliveryOfCashOnSettlement |  |
Underwriting discount included in delivery of cash | 5.0 he_EquityForwardTransactionUnderwritingDiscountIncludedInCashInExchangeOfPhysicalDeliveryOfSharesOnSettlement | 1.0 he_EquityForwardTransactionUnderwritingDiscountIncludedInCashInExchangeOfPhysicalDeliveryOfSharesOnSettlement | 1.3 he_EquityForwardTransactionUnderwritingDiscountIncludedInCashInExchangeOfPhysicalDeliveryOfSharesOnSettlement |  |
Cash in exchange of physical delivery of shares on settlement | $ 106 he_EquityForwardTransactionCashInExchangeOfPhysicalDeliveryOfSharesOnSettlement |  |  |  |

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- Definition Common Stock, Capital Shares Reserved for Future Issuance, Equity Forward Transaction + References No definition available. + Details
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- Definition Represents the amount of cash in exchange of physical delivery of shares to forward counterparty on settlement of equity forward transaction. + References No definition available. + Details
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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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- Definition Represents the amount of delivery of net shares to forward counterparty on settlement of equity forward transaction. + References No definition available. + Details
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- Definition Represents the forward sale price determined at the time of entering the equity forward transaction. + References No definition available. + Details
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- Definition Represents the initial fair value of common stock related to equity forward transaction. + References No definition available. + Details
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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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- Definition Represents the number of shares borrowed by the forward counterparty from third parties. + References No definition available. + Details
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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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- Definition Represents the number of shares in connection with underwriters exercising their over-allotment option under the equity forward transaction. + References No definition available. + Details
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- Definition Represents the underwriting discount on sale of common stock to underwriters. + References No definition available. + Details
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- Definition Represents the amount of underwriting discount included in delivery of cash in exchange of physical delivery of shares to forward counterparty on settlement of equity forward transaction. + References No definition available. + Details
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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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- Details
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- Definition Price of a single share of a number of saleable stocks of a company. + References No definition available. + Details
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