Share-based compensation - Share-Based Compensation Expense and Related Income Tax Benefit (Details) - USD ($) $ in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2025	Sep. 30, 2024	Sep. 30, 2025	Sep. 30, 2024
Share-based compensation
Share-based compensation expense (benefit)	$ (0.6)	$ (0.3)	$ 2.9	$ 2.2
Income tax benefit (expense)	(0.1)	0.0	0.4	0.2
Hawaiian Electric Company, Inc. and Subsidiaries
Share-based compensation
Share-based compensation expense (benefit)	(0.2)	(0.1)	1.6	1.1
Income tax benefit (expense)	$ 0.0	$ (0.1)	$ 0.3	$ 0.1