Share-based compensation - Share-Based Compensation Expense and Related Income Tax Benefit (Details) - USD ($) $ in Millions	3 Months Ended		6 Months Ended
	Jun. 30, 2025	Jun. 30, 2024	Jun. 30, 2025	Jun. 30, 2024
Share-based compensation
Share-based compensation expense	$ 2.8	$ 1.6	$ 3.5	$ 2.5
Income tax benefit	0.5	0.1	0.5	0.2
Hawaiian Electric Company, Inc. and Subsidiaries
Share-based compensation
Share-based compensation expense	1.3	0.8	1.8	1.2
Income tax benefit	$ 0.2	$ 0.1	$ 0.3	$ 0.1