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Summary of significant accounting policies - General, Cash, and PPE (Details) $ in Thousands |  |  | 1 Months Ended | 12 Months Ended
--- | --- | --- | --- | ---
Feb. 02, 2018 MWh MW | Nov. 24, 2017 MW | Sep. 30, 2020 MW | Dec. 31, 2021 USD ($) MWh branch MW | Dec. 31, 2020 USD ($) | Dec. 31, 2019
Significant Accounting Policies [Line Items] |  |  |  |  |  |
Number of branches |  |  |  | 42 |  |
Restricted cash | $ |  |  |  | $ 5,900 | $ 17,600 |
Oahu |  |  |  |  |  |
Significant Accounting Policies [Line Items] |  |  |  |  |  |
Number of branches |  |  |  | 29 |  |
Maui |  |  |  |  |  |
Significant Accounting Policies [Line Items] |  |  |  |  |  |
Number of branches |  |  |  | 6 |  |
Kauai |  |  |  |  |  |
Significant Accounting Policies [Line Items] |  |  |  |  |  |
Number of branches |  |  |  | 2 |  |
Hawaii |  |  |  |  |  |
Significant Accounting Policies [Line Items] |  |  |  |  |  |
Number of branches |  |  |  | 4 |  |
Molokai |  |  |  |  |  |
Significant Accounting Policies [Line Items] |  |  |  |  |  |
Number of branches |  |  |  | 1 |  |
Hawaiian Electric Company, Inc. and Subsidiaries |  |  |  |  |  |
Significant Accounting Policies [Line Items] |  |  |  |  |  |
Composite annual depreciation rate |  |  |  | 3.20% | 3.20% | 3.20%
Minimum |  |  |  |  |  |
Significant Accounting Policies [Line Items] |  |  |  |  |  |
Estimated useful life under production plant (in years) |  |  |  | 16 years |  |
Estimated useful life under transmission and distribution plant (in years) |  |  |  | 10 years |  |
Estimated useful life under general plant (in years) |  |  |  | 5 years |  |
Maximum |  |  |  |  |  |
Significant Accounting Policies [Line Items] |  |  |  |  |  |
Estimated useful life under production plant (in years) |  |  |  | 51 years |  |
Estimated useful life under transmission and distribution plant (in years) |  |  |  | 79 years |  |
Estimated useful life under general plant (in years) |  |  |  | 50 years |  |
Hamakua Energy |  |  |  |  |  |
Significant Accounting Policies [Line Items] |  |  |  |  |  |
Power produced by power plants (in megawatts) | MW |  | 60 |  | 60 |  |
Mauo, LLC |  |  |  |  |  |
Significant Accounting Policies [Line Items] |  |  |  |  |  |
Power produced by power plants (in megawatts) | MW | 8.6 |  |  | 8.6 |  |
Power storage capacity (in megawatt hours) | MWh | 42.3 |  |  | 42.3 |  |
American Savings Bank (ASB) |  |  |  |  |  |
Significant Accounting Policies [Line Items] |  |  |  |  |  |
Reserve requirement | $ |  |  |  |  | $ 0 |
Hawaiian Electric Company, Inc. and Subsidiaries |  |  |  |  |  |
Significant Accounting Policies [Line Items] |  |  |  |  |  |
Restricted cash | $ |  |  |  | $ 3,089 | $ 15,966 |
Ka‘ie‘ie Waho Company, LLC |  |  |  |  |  |
Significant Accounting Policies [Line Items] |  |  |  |  |  |
Photovoltaic system power amount (in megawatts) | MW |  |  | 6 |  |  |

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