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COMMITMENTS, GUARANTEES AND CONTINGENCIES - Power Purchase Agreements (Details) - Square Butte PPA - USD ($) $ in Millions | 9 Months Ended | 12 Months Ended
--- | --- | ---
Sep. 30, 2025 | Sep. 30, 2024 | Dec. 31, 2024
Square Butte |  |  |
Power Purchase Agreements [Line Items] |  |  |
PPA counterparty total debt outstanding | $ 151.6 |  |
Square Butte | Minnesota Power |  |  |
Power Purchase Agreements [Line Items] |  |  |
Cost of power purchased | 62.4 | $ 66.0 |
Pro rata share of PPA counterparty interest expense | $ 3.1 | $ 3.8 |
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 | 46.00% |  | 41.00%

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- Definition 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. + References Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef -Topic 980 -SubTopic 10 -Name Accounting Standards Codification -Section S99 -Paragraph 1 -Subparagraph (SAB Topic 10.D) -Publisher FASB -URI https://asc.fasb.org/1943274/2147480114/980-10-S99-1 + Details
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