Share-based compensation - Share-Based Compensation Expense and Related Income Tax Benefit (Details) - USD ($) $ in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2024	Sep. 30, 2023	Sep. 30, 2024	Sep. 30, 2023
Share-based compensation
Share-based compensation expense (benefit)	$ 0.1	$ 2.4	$ 3.2	$ 8.3
Income tax benefit (expense)	0.0	0.5	0.3	1.6
Hawaiian Electric Company, Inc. and Subsidiaries
Share-based compensation
Share-based compensation expense (benefit)	(0.1)	0.7	1.1	2.5
Income tax benefit (expense)	$ (0.1)	$ 0.1	$ 0.1	$ 0.5