Income taxes - Additional Information (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2021	Dec. 31, 2020	Dec. 31, 2019
Income Tax Expense (Benefit) [Line Items]
Deferred tax assets, valuation allowance	$ 0.0	$ 0.0
Unrecognized tax benefits that would impact effective tax rate	10.2	11.6
Credit adjustments to interest expense on income taxes	0.2	(0.5)	$ 0.1
Amount of accrued interest related to uncertain tax positions	0.3	0.1
Hawaiian Electric Company, Inc. and Subsidiaries
Income Tax Expense (Benefit) [Line Items]
Unrecognized tax benefits that would impact effective tax rate	10.2	11.6
Credit adjustments to interest expense on income taxes	0.1	(0.3)	$ 0.1
Amount of accrued interest related to uncertain tax positions	$ 0.1	$ 0.1