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Regulatory matters - NDPSC (Details) - NDPSC - Montana-Dakota Utilities Co. - USD ($) $ in Millions | Oct. 31, 2025 | Jul. 15, 2025
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Electric Transmission |  |
Public Utilities, General Disclosures [Line Items] |  |
Transmission cost adjustment rider, requested revenue recovery |  | $ 6.3
Public utilities, requested rate increase (decrease), amount |  | 7.2
Capital investment in transmission projects, requested revenue recovery amount |  | $ 8.6
Natural gas distribution | Subsequent Event |  |
Public Utilities, General Disclosures [Line Items] |  |
Public utilities, interim rate increase (decrease), amount | $ 43.6 |
Public utilities, annual revenue increase (decrease) | $ 25.3 |

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