Income taxes - Narrative (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2023	Dec. 31, 2022	Dec. 31, 2021
Income Tax Expense (Benefit) [Line Items]
Deferred tax assets, valuation allowance	$ 0.0	$ 0.0
Unrecognized tax benefits that would impact effective tax rate	13.8	10.2
Credit adjustments to interest expense on income taxes	1.3	0.4	$ 0.2
Amount of accrued interest related to uncertain tax positions	1.9	0.6
Settlement with Taxing Authority
Income Tax Expense (Benefit) [Line Items]
Decrease in unrecognized tax benefits is reasonably possible	1.8
Lapse In Statute Of Limitations
Income Tax Expense (Benefit) [Line Items]
Decrease in unrecognized tax benefits is reasonably possible	1.3
Minimum | Settlement with Taxing Authority
Income Tax Expense (Benefit) [Line Items]
Estimate of unrecognized tax benefits related to tax credit claims	0.0
Maximum | Settlement with Taxing Authority
Income Tax Expense (Benefit) [Line Items]
Estimate of unrecognized tax benefits related to tax credit claims	10.0
Hawaiian Electric Company, Inc. and Subsidiaries
Income Tax Expense (Benefit) [Line Items]
Credit adjustments to interest expense on income taxes	0.1	0.1	$ 0.1
Amount of accrued interest related to uncertain tax positions	$ 0.4	$ 0.2