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Electric utility segment - Rate proceedings (Details) - USD ($) $ in Millions |  |  |  |  |  |  |  | 1 Months Ended
--- | --- | --- | --- | --- | --- | --- | --- | ---
Mar. 18, 2019 | Dec. 14, 2018 | Aug. 09, 2018 | Jun. 22, 2018 | May 01, 2018 | Feb. 16, 2018 | Aug. 21, 2017 | Aug. 31, 2017
Regulatory Projects and Legal Obligations [Line Items] |  |  |  |  |  |  |  |
Interim rate increases |  |  |  |  |  | $ 36.0 |  |
Rate increase before tax act impact |  |  |  | $ 37.7 |  |  |  |
Tax act impact on rate, amount |  |  |  | $ 38.3 |  |  |  |
General rate increase, revenue, calculation assumptions, rate of return, ROACE |  |  |  | 9.50% |  |  |  |
General rate increase, revenue, calculation assumptions, rate of return |  |  |  | 7.57% |  |  |  |
Risk sharing percentage, ratepayer |  |  |  | 98.00% |  |  |  |
Risk sharing percentage, utility |  |  |  | 2.00% |  |  |  |
Maximum exposure cap |  |  |  | $ 2.5 |  |  |  |
Maui Electric |  |  |  |  |  |  |  |
Regulatory Projects and Legal Obligations [Line Items] |  |  |  |  |  |  |  |
Risk sharing percentage, ratepayer | 98.00% |  |  |  |  |  |  |
Risk sharing percentage, utility | 2.00% |  |  |  |  |  |  |
Maximum exposure cap | $ 0.6 |  |  |  |  |  |  |
Interim general rate increase granted |  |  | $ 12.2 |  |  |  |  |
Effective interest rate of return |  |  | 7.43% |  |  |  |  |
Stipulated ROACE rate |  |  | 9.50% |  |  |  |  |
Common equity capitalization rate |  |  | 57.00% |  |  |  |  |
Rate Base |  |  | $ 454.0 |  |  |  |  |
HELCO |  |  |  |  |  |  |  |
Regulatory Projects and Legal Obligations [Line Items] |  |  |  |  |  |  |  |
Interim rate increases |  |  |  |  | $ (9.9) |  |  |
Interim general rate increase granted |  |  |  |  |  |  | $ 9.9 |
Stipulated ROACE rate |  | 10.50% |  |  |  |  |  | 9.50%
General rate increase, revenue |  | $ 13.4 |  |  |  |  |  |
General rate increase, revenue, percent |  | 3.40% |  |  |  |  |  |
Assumptions on rate of return, return on average common equity, percentage decrease |  | 8.30% |  |  |  |  |  |

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