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Summary of significant accounting policies - Depreciation (Details) $ in Millions |  |  | 12 Months Ended
--- | --- | --- | ---
Feb. 02, 2018 MWh MW | Nov. 24, 2017 MW | Dec. 31, 2018 USD ($) MW | Dec. 31, 2017 USD ($) | Dec. 31, 2016
Depreciation |  |  |  |  |
Composite annual depreciation rate |  |  | 3.20% | 3.20% | 3.20%
Minimum |  |  |  |  |
Depreciation |  |  |  |  |
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 |  |  |  |  |
Depreciation |  |  |  |  |
Estimated useful life under production plant (in years) |  |  | 88 years |  |
Estimated useful life under transmission and distribution plant (in years) |  |  | 79 years |  |
Estimated useful life under general plant (in years) |  |  | 65 years |  |
Hamakua Energy Partners, L.P. (HEP) |  |  |  |  |
Depreciation |  |  |  |  |
Power produced by power plants (in megawatts) |  | 60 | 60 |  |
Mauo, LLC |  |  |  |  |
Depreciation |  |  |  |  |
Power produced by power plants (in megawatts) | 8.6 |  |  |  |
Power storage capacity (in megawatt hours) | MWh | 42.3 |  |  |  |
American Savings Bank (ASB) |  |  |  |  |
Depreciation |  |  |  |  |
Reserve requirement | $ |  |  | $ 28.1 | $ 17.9 |

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