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Commitments, Guarantees and Contingencies - Power Purchase Agreements (Details) $ in Millions | 3 Months Ended
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Mar. 31, 2018 USD ($) MW | Mar. 31, 2017 USD ($)
Square Butte [Member] | Square Butte PPA [Member] |  |
Power Purchase Agreements [Line Items] |  |
PPA Counterparty Total Debt Outstanding | $ 314.5 |
Cost of Power Purchased | 17.3 | $ 20.3
Pro Rata Share of PPA Counterparty Interest Expense | $ 2.3 | $ 2.3
Square Butte [Member] | Square Butte PPA [Member] | Square Butte Coal-fired Unit [Member] |  |
Power Purchase Agreements [Line Items] |  |
Generating Capacity Counterparty Owned (MW) | MW | 455 |
Expected Output Entitlement | 50.00% |
Minnkota Power [Member] | Square Butte PPA [Member] | Minnkota Power PSA [Member] | Square Butte Coal-fired Unit [Member] |  |
Power Purchase Agreements [Line Items] |  |
Expected Output Entitlement | 28.00% | 28.00%
Oconto Electric Cooperative [Member] | Oconto Electric Cooperative PSA [Member] |  |
Power Purchase Agreements [Line Items] |  |
Energy and Capacity Expected to be Provided (MW) | MW | 25 |

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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://www.xbrl.org/2003/role/presentationRef -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://www.xbrl.org/2003/role/presentationRef -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 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. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -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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