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Retirement benefits - Narrative (Details) - USD ($) $ in Millions | 3 Months Ended
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Mar. 31, 2025 | Mar. 31, 2024
Retirement benefits |  |
Contributions made to defined benefit plans | $ 1.0 | $ 0.0
Contributions expected to be paid in current year | 9.0 |
Expected payments for remainder of fiscal year | 3.0 | 2.0
Retirement benefits expense | $ 11.0 | 12.0
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 | $ 1.8 | 1.9
Cash contributions by the employer to defined contribution plan | 1.8 | 1.9
Hawaiian Electric Company, Inc. and Subsidiaries |  |
Retirement benefits |  |
Contributions made to defined benefit plans | 1.0 |
Contributions expected to be paid in current year | 9.0 |
Expected payments for remainder of fiscal year | 1.0 | 1.0
Retirement benefits expense | 11.0 | 11.0
Defined contribution plan, expenses recognized | 1.7 | 1.8
Cash contributions by the employer to defined contribution plan | $ 1.7 | $ 1.8

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