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Income taxes - Additional Information (Details) - USD ($) | 3 Months Ended | 12 Months Ended |
--- | --- | --- | ---
Dec. 31, 2017 | Dec. 31, 2017 | Dec. 31, 2016 | Dec. 31, 2015 | Dec. 31, 2014
Income Tax Expense (Benefit) [Line Items] |  |  |  |  |
Net tax expense due to impact of the Tax Act | $ 14,200,000 | $ 13,400,000 |  |  |
Effective income tax rate |  | 39.60% | 33.10% | 36.50% |
Unrecognized tax benefits | 4,000,000 | $ 4,000,000 | $ 3,800,000 | $ 3,600,000 | $ 0
Deferred tax assets, valuation allowance | 0 | 0 | 0 |  |
Net operating loss | 0 | 0 | 0 |  |
Unrecognized tax benefits that would impact effective tax rate | 500,000 | 500,000 | 0 |  |
Credit adjustments to interest expense on income taxes |  | 200,000 | 200,000 | $ 100,000 |
Amount of accrued interest related to uncertain tax positions | 500,000 | 500,000 | $ 300,000 |  |
Tax Act, decrease related to deferred tax assets |  | 271,500,000 |  |  |
Tax Act, Increase in regulatory liabilities |  | 284,900,000 |  |  |
Hawaiian Electric Company, Inc. and Subsidiaries |  |  |  |  |
Income Tax Expense (Benefit) [Line Items] |  |  |  |  |
Net tax expense due to impact of the Tax Act | 9,200,000 | $ 9,200,000 |  |  |
Effective income tax rate |  | 40.60% | 37.00% | 36.60% |
Unrecognized tax benefits | 3,500,000 | $ 3,500,000 | $ 3,800,000 | $ 3,600,000 | $ 0
Net operating loss | 0 | 0 | 9,158,000 |  |
Unrecognized tax benefits that would impact effective tax rate | 0 | 0 | 0 |  |
Credit adjustments to interest expense on income taxes |  | 80,000 | 30,000 | $ 100,000 |
Amount of accrued interest related to uncertain tax positions | 200,000 | 200,000 | $ 100,000 |  |
Tax Act, decrease related to deferred tax assets |  | 275,700,000 |  |  |
Hawaiian Electric Company, Inc. and Subsidiaries | Federal Tax Authority |  |  |  |  |
Income Tax Expense (Benefit) [Line Items] |  |  |  |  |
Net operating loss | 9,000,000 | 9,000,000 |  |  |
Unrecognized tax benefits | 3,000,000 | 3,000,000 |  |  |
Hawaiian Electric Company, Inc. and Subsidiaries | Federal Tax Authority | General Business Tax Credit Carryforward |  |  |  |  |
Income Tax Expense (Benefit) [Line Items] |  |  |  |  |
Federal general business credit carryforward | $ 3,000,000 | $ 3,000,000 |  |  |

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- Definition Tax Cuts and Jobs Act of 2017, Incomplete Accounting, Change in Tax Rate, Decrease In Deferred Tax Liability + References No definition available. + Details
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- Definition Tax Cuts and Jobs Act of 2017, Incomplete Accounting, Change in Tax Rate, Increase In Regulatory Liabilities + References No definition available. + Details
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- Definition Amount before allocation of valuation allowances of deferred tax asset attributable to deductible operating loss carryforwards. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 740 -SubTopic 10 -Section 25 -Paragraph 20 -Subparagraph (b) -URI http://asc.fasb.org/extlink&oid=68052875&loc=d3e28680-109314 Reference 2: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 740 -SubTopic 10 -Section 50 -Paragraph 6 -URI http://asc.fasb.org/extlink&oid=65882285&loc=d3e32621-109319 Reference 3: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 740 -SubTopic 10 -Section 50 -Paragraph 8 -URI http://asc.fasb.org/extlink&oid=65882285&loc=d3e32632-109319 + Details
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- Definition Amount of deferred tax assets for which it is more likely than not that a tax benefit will not be realized. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 740 -SubTopic 10 -Section 50 -Paragraph 2 -Subparagraph (c) -URI http://asc.fasb.org/extlink&oid=65882285&loc=d3e32537-109319 + Details
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- Definition Amount of operating loss carryforward, before tax effects, available to reduce future taxable income under enacted tax laws. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 740 -SubTopic 10 -Section 50 -Paragraph 3 -Subparagraph (a) -URI http://asc.fasb.org/extlink&oid=65882285&loc=d3e32559-109319 + Details
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- Definition Amount of unrecognized tax benefits. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 740 -SubTopic 10 -Section 50 -Paragraph 15A -Subparagraph (a) -URI http://asc.fasb.org/extlink&oid=65882285&loc=SL6600010-109319 Reference 2: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Glossary Unrecognized Tax Benefit -URI http://asc.fasb.org/extlink&oid=6527854 + Details
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- Definition Amount of interest expense accrued for an underpayment of income taxes. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 740 -SubTopic 10 -Section 50 -Paragraph 15 -Subparagraph (c) -URI http://asc.fasb.org/extlink&oid=65882285&loc=d3e32718-109319 + Details
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