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Electric utility segment - Utility projects (Details) - USD ($) $ in Millions |  | 1 Months Ended | 3 Months Ended |
--- | --- | --- | --- | ---
Jun. 10, 2019 | Nov. 30, 2020 | Jan. 31, 2020 | Mar. 31, 2021 | Dec. 31, 2020
ERP/EAM Implementation Project |  |  |  |  |
Public Utilities, General Disclosures [Line Items] |  |  |  |  |
ERP/EAM project service period (in years) | 12 years |  |  |  |
Future O&M expense reductions | $ 150.0 |  |  |  |
Future cost avoidance related to capital cost and tax costs | $ 96.0 |  |  |  |
Hawaiian Electric (parent only) | PCB Contamination |  |  |  |  |
Public Utilities, General Disclosures [Line Items] |  |  |  |  |
Valuation allowances and reserves |  |  |  | $ 9.8 |
Hawaiian Electric (parent only) | ERP/EAM Implementation Project |  |  |  |  |
Public Utilities, General Disclosures [Line Items] |  |  |  |  |
ERP/EAM project service period (in years) |  | 12 years |  |  |
Regulatory liability for O&M expense reductions |  |  |  | 6.5 |
HELCO | ERP/EAM Implementation Project |  |  |  |  |
Public Utilities, General Disclosures [Line Items] |  |  |  |  |
ERP/EAM project service period (in years) |  |  | 12 years |  |
Regulatory liability for O&M expense reductions |  |  |  | 1.9 |
Regulatory liability to be flowed through to customers |  |  |  |  | $ 1.6
Hawaiian Electric Company, Inc. and Subsidiaries | ERP/EAM Implementation Project |  |  |  |  |
Public Utilities, General Disclosures [Line Items] |  |  |  |  |
Regulatory liability for O&M expense reductions |  |  |  | 11.2 |
Maui Electric |  |  |  |  |
Public Utilities, General Disclosures [Line Items] |  |  |  |  |
Additional accrued investigation and estimated cleanup costs |  |  |  | 2.7 |
Maui Electric | ERP/EAM Implementation Project |  |  |  |  |
Public Utilities, General Disclosures [Line Items] |  |  |  |  |
Regulatory liability for O&M expense reductions |  |  |  | $ 2.8 |
Regulatory liability, amortization period |  |  |  | 5 years |
Regulatory liability to be flowed through to customers |  |  |  |  | $ 2.3

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