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Income taxes - Schedule of Changes in Unrecognized Tax Benefits (Details) - USD ($)
$ in Millions
 12 Months Ended
Dec. 31, 2023
Dec. 31, 2022
Dec. 31, 2021
Changes in total unrecognized tax benefits      
Unrecognized tax benefits, at the beginning of the period $ 30.6 $ 17.1 $ 12.7
Additions based on tax positions taken during the year 0.5 19.0 2.8
Reductions based on tax positions taken during the year 0.0 (3.5) (0.5)
Additions for tax positions of prior years 3.8 0.6 7.6
Reductions for tax positions of prior years (7.2) (2.6) (5.5)
Lapses of statute of limitations 0.0 0.0 0.0
Settlement 0.0 0.0 0.0
Unrecognized tax benefits, at the end of the period 27.7 30.6 17.1
Hawaiian Electric Company, Inc. and Subsidiaries      
Changes in total unrecognized tax benefits      
Unrecognized tax benefits, at the beginning of the period 11.7 11.6 12.7
Additions based on tax positions taken during the year 0.3 0.1 0.3
Reductions based on tax positions taken during the year 0.0 0.0 0.0
Additions for tax positions of prior years 3.7 0.2 0.2
Reductions for tax positions of prior years (0.1) (0.2) (1.6)
Lapses of statute of limitations 0.0 0.0 0.0
Settlement 0.0 0.0 0.0
Unrecognized tax benefits, at the end of the period $ 15.6 $ 11.7 $ 11.6