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Commitments, Guarantees and Contingencies - Power Purchase Agreements (Details) $ in Millions | 9 Months Ended
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Sep. 30, 2018 USD ($) MW | Sep. 30, 2017 USD ($)
Square Butte [Member] | Square Butte PPA [Member] |  |
Power Purchase Agreements [Line Items] |  |
PPA Counterparty Total Debt Outstanding | $ | $ 308.4 |
Square Butte [Member] | Square Butte PPA [Member] | Minnesota Power [Member] |  |
Power Purchase Agreements [Line Items] |  |
Cost of Power Purchased | $ | 57.9 | $ 60.6
Pro Rata Share of PPA Counterparty Interest Expense | $ | $ 6.9 | $ 7.1
Square Butte [Member] | Square Butte PPA [Member] | Square Butte Coal-fired Unit [Member] |  |
Power Purchase Agreements [Line Items] |  |
Generating Capacity Counterparty Owned (MW) | 455 |
Square Butte [Member] | Square Butte PPA [Member] | Square Butte Coal-fired Unit [Member] | Minnesota Power [Member] |  |
Power Purchase Agreements [Line Items] |  |
Expected Output Entitlement | 50.00% |
Minnkota Power [Member] | Square Butte PPA [Member] | Square Butte Coal-fired Unit [Member] | Minnesota Power [Member] | Minnkota Power PSA [Member] |  |
Power Purchase Agreements [Line Items] |  |
Expected Output Entitlement | 28.00% | 28.00%
Oconto Electric Cooperative [Member] | Minnesota Power [Member] | Oconto Electric Cooperative PSA [Member] |  |
Power Purchase Agreements [Line Items] |  |
Energy and Capacity Expected to be Provided (MW) | 25 |
Calpine Corporation [Member] | Minnesota Power [Member] | Calpine Corporation Power Purchase Agreement [Member] |  |
Power Purchase Agreements [Line Items] |  |
Capacity Purchased (MW) | 25 |
Basin Electric Power Cooperative [Member] | Minnesota Power [Member] | Basin Electric Cooperative PSA [Member] |  |
Power Purchase Agreements [Line Items] |  |
Capacity Sale (MW) | 100 |
Cypress Creek Renewables [Member] | Cypress Creek Renewables PPA [Member] | Blanchard Solar Energy Facility [Member] | Minnesota Power [Member] |  |
Power Purchase Agreements [Line Items] |  |
Electric Energy and Capacity Purchased (MW) | 10 |
Contract Length (Years) | 25 years |

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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=6499423&loc=d3e659339-123030 + Details
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- Definition The allocable portion of interest included in charges under a long-term contract to purchase electric power. + 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=6499423&loc=d3e659339-123030 + Details
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