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

[Line Items] for Income Tax Expense (Benefit) [Table]

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Tax Cuts and Jobs Act of 2017, Incomplete Accounting, Change in Tax Rate, Decrease In Deferred Tax Liability

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Tax Cuts and Jobs Act of 2017, Incomplete Accounting, Change in Tax Rate, Provisional Income Tax Expense (Benefit)

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Amount before allocation of valuation allowances of deferred tax asset attributable to deductible operating loss carryforwards.

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Amount of deferred tax assets for which it is more likely than not that a tax benefit will not be realized.

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Percentage of current income tax expense (benefit) and deferred income tax expense (benefit) pertaining to continuing operations.

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Amount of operating loss carryforward, before tax effects, available to reduce future taxable income under enacted tax laws.

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Amount of unrecognized tax benefits.

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Amount of interest expense accrued for an underpayment of income taxes.

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Amount of interest expense for an underpayment of income taxes.

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The total amount of unrecognized tax benefits that, if recognized, would affect the effective tax rate.

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