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Electric utility segment - Commitments and contingencies (Details) $ in Millions |  |  | 1 Months Ended | 12 Months Ended |
--- | --- | --- | --- | --- | ---
Nov. 06, 2020 | Jun. 10, 2019 USD ($) | Jan. 31, 2020 | Jul. 31, 2018 MW | Jun. 30, 2015 MW | May 31, 2012 MW | Mar. 31, 1988 MW | Dec. 31, 1988 MW | Sep. 30, 2020 USD ($)
Regulatory Projects and Legal Obligations [Line Items] |  |  |  |  |  |  |  |  |
ERP/EAM project service period (in years) |  | 12 years |  |  |  |  |  |  |
Project costs, accrued carrying costs once put into service | $ |  |  |  |  |  |  |  |  | $ 59.5
Project costs, amortization | $ |  |  |  |  |  |  |  |  | 0.5
Future O&M expense reductions | $ |  | $ 150.0 |  |  |  |  |  |  |
Future cost avoidance related to capital cost and tax costs | $ |  | $ 96.0 |  |  |  |  |  |  |
AES Hawaii |  |  |  |  |  |  |  |  |
Regulatory Projects and Legal Obligations [Line Items] |  |  |  |  |  |  |  |  |
Purchase commitment, period (in years) |  |  |  |  |  |  | 30 years |  |
Minimum power volume required (in megawatts) | MW |  |  |  |  |  |  | 180 |  |
Additional capacity requirement (in megawatts) | MW |  |  |  |  | 9 |  |  |  |
Hu Honua Bioenergy, LLC |  |  |  |  |  |  |  |  |
Regulatory Projects and Legal Obligations [Line Items] |  |  |  |  |  |  |  |  |
Minimum power volume required (in megawatts) | MW |  |  |  |  |  | 21.5 |  |  |
Molokai New Energy Partners |  |  |  |  |  |  |  |  |
Regulatory Projects and Legal Obligations [Line Items] |  |  |  |  |  |  |  |  |
Total contracted PV capacity (in megawatts) | MW |  |  |  | 4.88 |  |  |  |  |
Total contracted battery energy storage system capacity | MW |  |  |  | 3 |  |  |  |  |
Maximum power volume to be delivered (in megawatts) | MW |  |  |  | 2.64 |  |  |  |  |
Hawaiian Electric (parent only) | Subsequent Event |  |  |  |  |  |  |  |  |
Regulatory Projects and Legal Obligations [Line Items] |  |  |  |  |  |  |  |  |
ERP/EAM project service period (in years) | 12 years |  |  |  |  |  |  |  |
Hawaiian Electric (parent only) | PCB Contamination |  |  |  |  |  |  |  |  |
Environmental regulation |  |  |  |  |  |  |  |  |
Valuation allowances and reserves | $ |  |  |  |  |  |  |  |  | 4.6
HELCO |  |  |  |  |  |  |  |  |
Regulatory Projects and Legal Obligations [Line Items] |  |  |  |  |  |  |  |  |
ERP/EAM project service period (in years) |  |  | 12 years |  |  |  |  |  |
Maui Electric |  |  |  |  |  |  |  |  |
Environmental regulation |  |  |  |  |  |  |  |  |
Additional accrued investigation and estimated cleanup costs | $ |  |  |  |  |  |  |  |  | 2.7
ERP/EAM Implementation Project | Hawaiian Electric Company, Inc. and Subsidiaries |  |  |  |  |  |  |  |  |
Regulatory Projects and Legal Obligations [Line Items] |  |  |  |  |  |  |  |  |
Regulatory liability for O&M expense reductions | $ |  |  |  |  |  |  |  |  | $ 9.7
Kalaeloa | Hawaiian Electric (parent only) |  |  |  |  |  |  |  |  |
Regulatory Projects and Legal Obligations [Line Items] |  |  |  |  |  |  |  |  |
Increased power purchase commitment capacity (in megawatts) | MW |  |  |  |  |  |  |  | 208 |

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