Income taxes - Changes in Unrecognized Tax Benefits (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
Changes in total unrecognized tax benefits
Unrecognized tax benefits, at the beginning of the period	$ 2.1	$ 4.0	$ 3.8
Additions based on tax positions taken during the year	0.5	0.3	0.9
Reductions based on tax positions taken during the year	0.0	0.0	(0.2)
Additions for tax positions of prior years	0.1	0.1	0.0
Reductions for tax positions of prior years	(0.2)	(0.1)	(0.5)
Lapses of statute of limitations	(0.3)	(2.2)	0.0
Unrecognized tax benefits, at the end of the period	2.2	2.1	4.0
Hawaiian Electric Company, Inc. and Subsidiaries
Changes in total unrecognized tax benefits
Unrecognized tax benefits, at the beginning of the period	1.6	3.5	3.8
Additions based on tax positions taken during the year	0.5	0.3	0.4
Reductions based on tax positions taken during the year	0.0	0.0	(0.2)
Additions for tax positions of prior years	0.1	0.1	0.0
Reductions for tax positions of prior years	(0.2)	(0.1)	(0.5)
Lapses of statute of limitations	(0.3)	(2.2)	0.0
Unrecognized tax benefits, at the end of the period	$ 1.7	$ 1.6	$ 3.5