Retirement benefits - Narrative (Details) - USD ($) $ in Millions	9 Months Ended
	Sep. 30, 2024	Sep. 30, 2023
Retirement benefits
Contributions made to defined benefit plans	$ 5.0	$ 6.0
Contributions expected to be paid in current year	9.0	8.0
Expected payments for remainder of fiscal year	3.0
Retirement benefits expense	$ 34.0	33.0
Number of years for which regulatory asset/liability for each utility will be amortized, beginning with respective utility's next rate case (in years)	5 years
Defined contribution plan, expenses recognized	$ 8.9	7.0
Cash contributions by the employer to defined contribution plan	8.9	7.6
Hawaiian Electric Company, Inc. and Subsidiaries
Retirement benefits
Contributions made to defined benefit plans	5.0	6.0
Contributions expected to be paid in current year	9.0	8.0
Expected payments for remainder of fiscal year	1.0
Retirement benefits expense	34.0	32.0
Defined contribution plan, expenses recognized	5.5	4.3
Cash contributions by the employer to defined contribution plan	$ 5.5	$ 4.3