Income taxes - Schedule of Changes in Unrecognized Tax Benefits (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2024	Dec. 31, 2023	Dec. 31, 2022
Changes in total unrecognized tax benefits
Unrecognized tax benefits, at the beginning of the period	$ 27.7	$ 30.6	$ 17.1
Additions based on tax positions taken during the year	0.7	0.5	19.0
Reductions based on tax positions taken during the year	(0.1)	0.0	(3.5)
Additions for tax positions of prior years	1.5	3.8	0.6
Reductions for tax positions of prior years	(4.6)	(7.2)	(2.6)
Lapses of statute of limitations	(1.9)	0.0	0.0
Settlement	(10.8)	0.0	0.0
Unrecognized tax benefits, at the end of the period	12.5	27.7	30.6
Hawaiian Electric Company, Inc. and Subsidiaries
Changes in total unrecognized tax benefits
Unrecognized tax benefits, at the beginning of the period	15.6	11.7	11.6
Additions based on tax positions taken during the year	0.0	0.3	0.1
Reductions based on tax positions taken during the year	0.0	0.0	0.0
Additions for tax positions of prior years	1.5	3.7	0.2
Reductions for tax positions of prior years	(0.6)	(0.1)	(0.2)
Lapses of statute of limitations	(0.5)	0.0	0.0
Settlement	(10.8)	0.0	0.0
Unrecognized tax benefits, at the end of the period	$ 5.2	$ 15.6	$ 11.7