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Regulatory matters - MTPSC (Details) - Natural gas distribution $ in Millions | Oct. 31, 2025 USD ($) | Oct. 20, 2025 USD ($) | Sep. 30, 2025 USD ($) | May 30, 2025 USD ($) | Jan. 14, 2025 USD ($) | Oct. 15, 2024 USD ($) | Jul. 15, 2024 USD ($) | Apr. 03, 2025 USD ($)
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MTPSC | Montana-Dakota Utilities Co. |  |  |  |  |  |  |  |
Public Utilities, General Disclosures [Line Items] |  |  |  |  |  |  |  |
Public utilities, requested rate increase (decrease), amount |  |  | $ 14.1 |  |  | $ 8.0 | $ 9.4 |
Public utilities, requested rate increase (decrease), percentage |  |  | 20.20% |  |  | 10.20% | 11.10% |
Public utilities, interim rate increase (decrease), amount |  |  |  |  | $ 7.7 |  |  |
Public utilities, annual revenue increase (decrease) |  |  |  |  |  |  |  | $ 7.3
Public utilities, annual revenue increase (decrease), percentage |  |  |  |  |  |  |  | 0.086
IPUC | Intermountain Gas Company |  |  |  |  |  |  |  |
Public Utilities, General Disclosures [Line Items] |  |  |  |  |  |  |  |
Public utilities, requested rate increase (decrease), amount |  |  |  | $ 26.5 |  |  |  |
Public utilities, requested rate increase (decrease), percentage |  |  |  | 8.60% |  |  |  |
IPUC | Intermountain Gas Company | Subsequent Event |  |  |  |  |  |  |  |
Public Utilities, General Disclosures [Line Items] |  |  |  |  |  |  |  |
Public utilities, requested rate increase (decrease), percentage |  | 4.20% |  |  |  |  |  |
Public utilities, annual revenue increase (decrease) |  | $ 13.0 |  |  |  |  |  |
SDPUC | Montana-Dakota Utilities Co. | Subsequent Event |  |  |  |  |  |  |  |
Public Utilities, General Disclosures [Line Items] |  |  |  |  |  |  |  |
Public utilities, interim rate increase (decrease), amount | $ 1.5 |  |  |  |  |  |  |
Public utilities, annual revenue increase (decrease) | $ 1.1 |  |  |  |  |  |  |

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