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Segment financial information - Narrative (Details) $ in Millions |  |  | 3 Months Ended | 9 Months Ended | 12 Months Ended
--- | --- | --- | --- | --- | ---
Dec. 31, 2024 segment | Dec. 30, 2024 USD ($) | Mar. 31, 2025 USD ($) | Sep. 30, 2025 USD ($) subsidiary | Dec. 30, 2024 USD ($) segment | Dec. 31, 2017 MW
Segment financial information |  |  |  |  |  |
Number of reportable segments | segment | 1 |  |  |  | 2 |
Number of operating segments | segment | 1 |  |  |  | 2 |
GLST1, LLC |  |  |  |  |  |
Segment financial information |  |  |  |  |  |
Membership interest |  |  | 60.00% |  |  |
Investment in unconsolidated affiliate |  |  |  | $ 287.3 |  |
Hamakua Energy |  |  |  |  |  |
Segment financial information |  |  |  |  |  |
Power produced by power plants (in megawatts) | MW |  |  |  |  |  | 60
Hamakua Holdings, LLC |  |  |  |  |  |
Segment financial information |  |  |  |  |  |
Number of wholly-owned subsidiaries | subsidiary |  |  |  | 2 |  |
GLST1, LLC |  |  |  |  |  |
Segment financial information |  |  |  |  |  |
First settlement payment |  |  |  | $ 479.0 |  |
American Savings Bank (ASB) | ASB Hawaii, Inc |  |  |  |  |  |
Segment financial information |  |  |  |  |  |
Minority ownership percentage |  |  |  | 9.90% |  |
Noncontrolling ownership interest |  |  |  | $ 44.6 |  |
Discontinued Operations, Disposed of by Sale | American Savings Bank (ASB) |  |  |  |  |  |
Segment financial information |  |  |  |  |  |
Ownership percentage disposed | 90.10% | 90.10% |  |  |  |
Consideration amount |  | $ 405.5 |  |  | $ 405.5 |
Disposal Group, Held-for-Sale, Not Discontinued Operations | Pacific Current |  |  |  |  |  |
Segment financial information |  |  |  |  |  |
Asset impairment charges |  |  |  | 0.2 |  |
Investment tax credit |  |  |  | $ 5.3 |  |
Disposal Group, Disposed of by Sale, Not Discontinued Operations | Hamakua Holdings, LLC |  |  |  |  |  |
Segment financial information |  |  |  |  |  |
Loss on sale |  |  | $ 13.2 |  |  |

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