Regulatory matters - WYPSC (Details) - WYPSC - Montana-Dakota Utilities Co. $ in Millions	Jun. 30, 2025 USD ($)	Oct. 31, 2024 USD ($)	May 09, 2025 USD ($)
Natural gas distribution
Public Utilities, General Disclosures [Line Items]
Public utilities, annual revenue increase (decrease)			$ 2.1
Public utilities, annual revenue increase (decrease), percentage			0.117
Natural gas distribution | Pending Rate Case
Public Utilities, General Disclosures [Line Items]
Public utilities, requested rate increase (decrease), amount		$ 2.6
Public utilities, requested rate increase (decrease), percentage		14.00%
Electric distribution | Pending Rate Case
Public Utilities, General Disclosures [Line Items]
Public utilities, requested rate increase (decrease), amount	$ 7.5
Public utilities, requested rate increase (decrease), percentage	24.40%