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Commitments, Guarantees and Contingencies - Power Purchase Agreements (Details) MWh in Millions, $ in Millions | 12 Months Ended
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Dec. 31, 2021 USD ($) MWh MW | Dec. 31, 2020 USD ($) | Dec. 31, 2019 USD ($)
Square Butte [Member] | Square Butte PPA (expires 2026) [Member] |  |  |
Power Purchase Agreements [Line Items] |  |  |
PPA Counterparty Total Debt Outstanding | $ | $ 222.6 |  |
Square Butte [Member] | Square Butte PPA (expires 2026) [Member] | Square Butte Coal-fired Unit [Member] |  |  |
Power Purchase Agreements [Line Items] |  |  |
Generating Capacity Counterparty Owned (MW) | 455 |  |
Square Butte [Member] | Square Butte PPA (expires 2026) [Member] | Minnesota Power [Member] |  |  |
Power Purchase Agreements [Line Items] |  |  |
Cost of Purchased Power | $ | $ 82.4 | $ 79.5 | $ 82.7
Pro Rata Share of PPA Counterparty Interest Expense | $ | $ 5.8 | $ 7.1 | $ 8.3
Square Butte [Member] | Square Butte PPA (expires 2026) [Member] | Minnesota Power [Member] | Square Butte Coal-fired Unit [Member] |  |  |
Power Purchase Agreements [Line Items] |  |  |
Minnesota Power Output Entitlement (Percent) | 50.00% |  |
PPA Annual Debt Service Obligation (Percentage) | 50.00% |  |
Manitoba Hydro [Member] | Manitoba Hydro PPA (expires April 2022) [Member] | Minnesota Power [Member] | Minimum [Member] |  |  |
Power Purchase Agreements [Line Items] |  |  |
Output Being Purchased (MWh) | MWh | 1,000,000 |  |
Manitoba Hydro [Member] | Manitoba Hydro PPA (expires 2040) [Member] | Minnesota Power [Member] |  |  |
Power Purchase Agreements [Line Items] |  |  |
Output Being Purchased - Electric Energy (MW) | 133 |  |
Manitoba Hydro [Member] | Manitoba Hydro PPA (expires May 2035) [Member] | Minnesota Power [Member] |  |  |
Power Purchase Agreements [Line Items] |  |  |
Output Being Purchased - Electric Energy and Capacity (MW) | 250 |  |
Nobles 2 [Member] | Nobles 2 [Member] | Minnesota Power [Member] |  |  |
Power Purchase Agreements [Line Items] |  |  |
Output Being Purchased - Electric Energy and Capacity (MW) | 250 |  |
Oliver Wind [Member] | Oliver Wind I PPA [Member] | Minnesota Power [Member] |  |  |
Power Purchase Agreements [Line Items] |  |  |
Output Being Purchased - Electric Energy (MW) | 50 |  |
Oliver Wind [Member] | Oliver Wind II PPA [Member] | Minnesota Power [Member] |  |  |
Power Purchase Agreements [Line Items] |  |  |
Output Being Purchased - Electric Energy (MW) | 48 |  |
Basin Electric Power Cooperative [Member] | Basin Electric Cooperative PSA June 2022 - May 2023 [Member] | Minnesota Power [Member] |  |  |
Power Purchase Agreements [Line Items] |  |  |
Output Being Sold Electric - Capacity (MW) | 75 |  |
Basin Electric Power Cooperative [Member] | Basin Electric Cooperative PSA June 2023 - May 2025 [Member] | Minnesota Power [Member] |  |  |
Power Purchase Agreements [Line Items] |  |  |
Output Being Sold Electric - Capacity (MW) | 125 |  |
Minnkota Power [Member] | Square Butte PPA (expires 2026) [Member] | Minnkota Sales Agreement [Member] | Square Butte Coal-fired Unit [Member] |  |  |
Power Purchase Agreements [Line Items] |  |  |
Minnesota Power Output Entitlement (Percent) | 28.00% | 28.00% | 28.00%
Silver Bay Power [Member] | Minnesota Power [Member] | Silver Bay Power Self-Generation [Member] |  |  |
Power Purchase Agreements [Line Items] |  |  |
Energy Generation Owned (MW) | 90 |  |
Calpine Corporation [Member] | Calpine Corporation PPA [Member] | Minnesota Power [Member] |  |  |
Power Purchase Agreements [Line Items] |  |  |
Output Being Purchased - Electric Capacity (MW) | 25 |  |
Great River Energy [Member] | Great River Energy Power Sales Agreement (expires May 2025) | Minnesota Power [Member] |  |  |
Power Purchase Agreements [Line Items] |  |  |
Output Being Sold Electric - Capacity (MW) | 100 |  |
Basin [Member] | Basin Power Sales Agreement (expires May 2028) [Member] | Minnesota Power [Member] |  |  |
Power Purchase Agreements [Line Items] |  |  |
Output Being Sold Electric - Capacity (MW) | 100 |  |
Oconto Electric Cooperative [Member] | Oconto Electric Cooperative Power Sales Agreement [Member] | Minnesota Power [Member] |  |  |
Power Purchase Agreements [Line Items] |  |  |
Output Being Sold Electric - Energy and Capacity (MW) | 25 |  |
2024-01-01 [Member] | Square Butte [Member] | Square Butte PPA (expires 2026) [Member] | Minnesota Power [Member] | Square Butte Coal-fired Unit [Member] |  |  |
Power Purchase Agreements [Line Items] |  |  |
PPA Counterparty Annual Debt Service through 2026 | $ | $ 30.5 |  |
2021-01-01 [Member] | Square Butte [Member] | Square Butte PPA (expires 2026) [Member] | Minnesota Power [Member] | Square Butte Coal-fired Unit [Member] |  |  |
Power Purchase Agreements [Line Items] |  |  |
PPA Counterparty Annual Debt Service through 2026 | $ | 45.5 |  |
2025-01-01 [Member] | Square Butte [Member] | Square Butte PPA (expires 2026) [Member] | Minnesota Power [Member] | Square Butte Coal-fired Unit [Member] |  |  |
Power Purchase Agreements [Line Items] |  |  |
PPA Counterparty Annual Debt Service through 2026 | $ | 26.6 |  |
2026-01-01 | Square Butte [Member] | Square Butte PPA (expires 2026) [Member] | Minnesota Power [Member] | Square Butte Coal-fired Unit [Member] |  |  |
Power Purchase Agreements [Line Items] |  |  |
PPA Counterparty Annual Debt Service through 2026 | $ | 40.1 |  |
2022-01-01 [Member] | Square Butte [Member] | Square Butte PPA (expires 2026) [Member] | Minnesota Power [Member] | Square Butte Coal-fired Unit [Member] |  |  |
Power Purchase Agreements [Line Items] |  |  |
PPA Counterparty Annual Debt Service through 2026 | $ | 45.5 |  |
2023-01-01 [Member] | Square Butte [Member] | Square Butte PPA (expires 2026) [Member] | Minnesota Power [Member] | Square Butte Coal-fired Unit [Member] |  |  |
Power Purchase Agreements [Line Items] |  |  |
PPA Counterparty Annual Debt Service through 2026 | $ | $ 45.5 |  |

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- Definition Share of plant output being purchased in megawatthours under a long-term contract to purchase electricity from a production plant constructed and financed by a public utility district, cooperative, or other organization. + References No definition available. + Details
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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 -Publisher FASB -Name Accounting Standards Codification -Topic 980 -SubTopic 10 -Section S99 -Paragraph 1 -Subparagraph (SAB Topic 10.D) -URI http://asc.fasb.org/extlink&oid=122136240&loc=d3e659339-123030 + Details
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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. If the utility is obligated to pay certain minimum amounts which cover debt service requirements whether or not the plant is operating, the amount can be disclosed here. + References Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef -Publisher FASB -Name Accounting Standards Codification -Topic 980 -SubTopic 10 -Section S99 -Paragraph 1 -Subparagraph (SAB Topic 10.D) -URI http://asc.fasb.org/extlink&oid=122136240&loc=d3e659339-123030 + Details
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