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Retirement benefits - Narrative (Details) - USD ($) $ in Millions | 9 Months Ended
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Sep. 30, 2024 | Sep. 30, 2023
Retirement benefits |  |
Contributions made to defined benefit plans | $ 5.0 | $ 6.0
Contributions expected to be paid in current year | 9.0 | 8.0
Expected payments for remainder of fiscal year | 3.0 |
Retirement benefits expense | $ 34.0 | 33.0
Number of years for which regulatory asset/liability for each utility will be amortized, beginning with respective utility's next rate case (in years) | 5 years |
Defined contribution plan, expenses recognized | $ 8.9 | 7.0
Cash contributions by the employer to defined contribution plan | 8.9 | 7.6
Hawaiian Electric Company, Inc. and Subsidiaries |  |
Retirement benefits |  |
Contributions made to defined benefit plans | 5.0 | 6.0
Contributions expected to be paid in current year | 9.0 | 8.0
Expected payments for remainder of fiscal year | 1.0 |
Retirement benefits expense | 34.0 | 32.0
Defined contribution plan, expenses recognized | 5.5 | 4.3
Cash contributions by the employer to defined contribution plan | $ 5.5 | $ 4.3

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