Share-based compensation - Share-Based Compensation Expense and Related Income Tax Benefit (Details) - USD ($) $ in Millions	3 Months Ended		6 Months Ended
	Jun. 30, 2024	Jun. 30, 2023	Jun. 30, 2024	Jun. 30, 2023
Share-based compensation
Share-based compensation expense	$ 1.8	$ 3.9	$ 3.1	$ 5.9
Income tax benefit	0.2	0.8	0.3	1.1
Hawaiian Electric Company, Inc. and Subsidiaries
Share-based compensation
Share-based compensation expense	0.8	1.1	1.2	1.7
Income tax benefit	$ 0.1	$ 0.2	$ 0.1	$ 0.4