Commitments, Guarantees and Contingencies - Power Purchase Agreements (Details) - Square Butte PPA - USD ($) $ in Millions	9 Months Ended
Sep. 30, 2023	Sep. 30, 2022
Square Butte
Power Purchase Agreements [Line Items]
PPA counterparty total debt outstanding	$ 176.9
Square Butte \| Minnesota Power
Power Purchase Agreements [Line Items]
Cost of power purchased	64.8	$ 63.8
Pro rata share of PPA counterparty interest expense	$ 4.1	$ 3.7
Square Butte \| Minnesota Power \| Square Butte Coal-Fired Unit
Power Purchase Agreements [Line Items]
Expected output entitlement	50.00%
Minnkota Power \| Minnesota Power \| Square Butte Coal-Fired Unit \| Minnkota Power PSA
Power Purchase Agreements [Line Items]
Expected output entitlement	37.00%	32.00%

Under long-term contracts with public utility districts, cooperatives or other organizations, a utility company receives a portion of the output of a production plant constructed and financed by the district or cooperative. The utility has only a nominal or no investment at all in the plant but pays a proportionate part of the plant's costs, including debt service.

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- Definition

The allocable portion of interest included in charges under a long-term contract to purchase electric power.

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Share of plant output being purchased under a long-term contract to purchase electricity from a production plant constructed and financed by a public utility district, cooperative, or other organization.

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- Definition

Amount of operating expense for purchased power procured under arrangements extending beyond one year or the normal operating cycle, if longer, by regulated operation.

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