Share-based compensation - Schedule of Share-based Compensation Expense and Related Income Tax Benefit (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2024	Dec. 31, 2023	Dec. 31, 2022
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Share-based compensation expense	$ 3.6	$ 6.8	$ 7.5
Income tax benefit	0.3	1.4	1.5
Hawaiian Electric Company, Inc. and Subsidiaries
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Share-based compensation expense	1.8	3.3	3.0
Income tax benefit	$ 0.2	$ 0.8	$ 0.7