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Commitments, Guarantees and Contingencies - Power Purchase Agreements (Details) - Square Butte - Square Butte PPA $ in Millions | 12 Months Ended
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Dec. 31, 2023 USD ($) MW | Dec. 31, 2022 USD ($) | Dec. 31, 2021 USD ($)
Power Purchase Agreements [Line Items] |  |  |
Ppa counterparty total debt outstanding | $ 171.8 |  |
Minnesota Power |  |  |
Power Purchase Agreements [Line Items] |  |  |
Cost of purchased power | 86.2 | $ 82.7 | $ 82.4
Pro rata share of ppa counterparty interest expense | $ 5.5 | $ 5.1 | $ 5.8
Square Butte Coal-fired Unit |  |  |
Power Purchase Agreements [Line Items] |  |  |
Generating capacity counterparty owned (MW) | MW | 455 |  |
Square Butte Coal-fired Unit | Minnesota Power |  |  |
Power Purchase Agreements [Line Items] |  |  |
Expected output entitlement | 50.00% |  |
Ppa annual debt service obligation | 50.00% |  |
2024-01-01 | Square Butte Coal-fired Unit | Minnesota Power |  |  |
Power Purchase Agreements [Line Items] |  |  |
Ppa counterparty annual debt service through 2027 | $ 33.5 |  |
2025-01-01 | Square Butte Coal-fired Unit | Minnesota Power |  |  |
Power Purchase Agreements [Line Items] |  |  |
Ppa counterparty annual debt service through 2027 | 29.5 |  |
2026-01-01 | Square Butte Coal-fired Unit | Minnesota Power |  |  |
Power Purchase Agreements [Line Items] |  |  |
Ppa counterparty annual debt service through 2027 | 29.6 |  |
2027-01-01 | Square Butte Coal-fired Unit | Minnesota Power |  |  |
Power Purchase Agreements [Line Items] |  |  |
Ppa counterparty annual debt service through 2027 | $ 11.9 |  |

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