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Electric utility segment - Commitments and contingencies (Details) $ in Millions |  | 1 Months Ended | 12 Months Ended |
--- | --- | --- | --- | ---
Jun. 10, 2019 USD ($) | Jan. 31, 2020 | Jun. 30, 2015 MW | May 31, 2012 MW | Mar. 31, 1988 MW | Dec. 31, 1988 MW | Mar. 31, 2020 USD ($)
Regulatory Projects and Legal Obligations [Line Items] |  |  |  |  |  |  |
ERP/EAM project service period (in years) | 12 years | 12 years |  |  |  |  |
Project costs, accrued carrying costs once put into service |  |  |  |  |  |  | $ 59.4
Project costs, amortization |  |  |  |  |  |  | 0.2
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 |  |  |
Hawaiian Electric (parent only) | PCB Contamination |  |  |  |  |  |  |
Environmental regulation |  |  |  |  |  |  |
Valuation allowances and reserves |  |  |  |  |  |  | 4.8
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 |  |  |  |  |  |  | $ 4.8
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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