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Income taxes (Details) | 3 Months Ended | 6 Months Ended
--- | --- | ---
Mar. 31, 2021 | Mar. 31, 2020 | Jun. 30, 2019
Income Tax Contingency [Line Items] |  |  |
Effective income tax, percent | 19.00% | 15.00% |
Federal income tax rate | 21.00% |  | 35.00%
Hawaiian Electric Company, Inc. and Subsidiaries |  |  |
Income Tax Contingency [Line Items] |  |  |
Effective income tax, percent | 20.00% | 18.00% |

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