Income taxes - Changes in Unrecognized Tax Benefits (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
Changes in total unrecognized tax benefits
Unrecognized tax benefits, at the beginning of the period	$ 2.2	$ 2.1	$ 4.0
Additions based on tax positions taken during the year	0.2	0.5	0.3
Reductions based on tax positions taken during the year	0.0	0.0	0.0
Additions for tax positions of prior years	11.6	0.1	0.1
Reductions for tax positions of prior years	(0.1)	(0.2)	(0.1)
Lapses of statute of limitations	(0.2)	(0.3)	(2.2)
Settlement	(1.0)	0.0	0.0
Unrecognized tax benefits, at the end of the period	12.7	2.2	2.1
Hawaiian Electric Company, Inc. and Subsidiaries
Changes in total unrecognized tax benefits
Unrecognized tax benefits, at the beginning of the period	1.7	1.6	3.5
Additions based on tax positions taken during the year	0.2	0.5	0.3
Reductions based on tax positions taken during the year	0.0	0.0	0.0
Additions for tax positions of prior years	11.6	0.1	0.1
Reductions for tax positions of prior years	(0.1)	(0.2)	(0.1)
Lapses of statute of limitations	(0.2)	(0.3)	(2.2)
Settlement	(0.5)	0.0	0.0
Unrecognized tax benefits, at the end of the period	$ 12.7	$ 1.7	$ 1.6