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 ($)	Apr. 03, 2025 USD ($)
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