Regulatory matters - NDPSC (Details) - NDPSC - Montana-Dakota Utilities Co. - USD ($) $ in Millions	Oct. 31, 2025	Jul. 15, 2025
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

Public Utilities, Annual Revenue Increase (Decrease)

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Public Utilities, Capital Investment In Transmission Projects, Requested Revenue Recovery, Amount

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Public Utilities, Transmission Cost Adjustment Rider, Requested Revenue Recovery, Amount

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Amount of public utility's interim rate increase (decrease) allowed while waiting final decision of regulatory agency.

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Amount of public utility's requested rate increase (decrease) with regulatory agency.

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