Subsequent events - Narrative (Details) - Senior Notes - Montana-Dakota Utilities Co. - Subsequent Event - USD ($) $ in Millions	Feb. 02, 2026	Oct. 28, 2025
Subsequent Events [Line Items]
Face amount		$ 250.0
Interest rate, stated percentage		5.96%
Proceeds from issuance of senior long-term debt		$ 150.0
Ratio of total debt to total capitalization		65.00%
Forecast
Subsequent Events [Line Items]
Proceeds from issuance of senior long-term debt	$ 100.0

Debt Covenant, Funded Debt To Total Capitalization, Ratio

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- Definition

Face (par) amount of debt instrument at time of issuance.

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- Definition

Contractual interest rate for funds borrowed, under the debt agreement.

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The cash inflow from a borrowing with the highest claim on the assets of the entity in case of bankruptcy or liquidation (with maturities initially due after one year or beyond the operating cycle, if longer).

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- Definition

Detail information of subsequent event by type. User is expected to use existing line items from elsewhere in the taxonomy as the primary line items for this disclosure, which is further associated with dimension and member elements pertaining to a subsequent event.

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