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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, 2018 USD ($) MWh MW | Dec. 31, 2017 USD ($) | Dec. 31, 2016 USD ($)
Square Butte [Member] | Square Butte PPA (expires 2026) [Member] |  |  |  |
Power Purchase Agreements [Line Items] |  |  |  |
PPA Counterparty Total Debt Outstanding | $ |  | $ 304.0 |  |
Square Butte [Member] | Square Butte PPA (expires 2026) [Member] | Minnesota Power [Member] |  |  |  |
Power Purchase Agreements [Line Items] |  |  |  |
Cost of Purchased Power | $ |  | 78.0 | $ 75.7 | $ 73.3
Pro Rata Share of PPA Counterparty Interest Expense | $ |  | $ 9.1 | $ 9.4 | $ 9.6
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] | Square Butte Coal-fired Unit [Member] | Minnesota Power [Member] |  |  |  |
Power Purchase Agreements [Line Items] |  |  |  |
Minnesota Power Output Entitlement (Percent) |  | 50.00% |  |
PPA Counterparty Annual Debt Service through 2023 | $ |  | $ 48.7 |  |
PPA Annual Debt Service Obligation (Percentage) |  | 50.00% |  |
Cypress Creek Renewables [Member] | Cypress Creek Renewables PPA [Member] | Blanchard Solar Energy Facility [Member] | Minnesota Power [Member] |  |  |  |
Power Purchase Agreements [Line Items] |  |  |  |
Output Being Purchased - Electric Energy and Capacity (MW) |  | 10 |  |
Contract Length (Years) |  | 25 years |  |
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 |  |
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 Beginning June 2017 (expires May 2020) [Member] | Minnesota Power [Member] |  |  |  |
Power Purchase Agreements [Line Items] |  |  |  |
Output Being Purchased - Electric Capacity (MW) |  | 50 |  |
Manitoba Hydro [Member] | Manitoba Hydro PPA (expires 2040) [Member] | Minnesota Power [Member] |  |  |  |
Power Purchase Agreements [Line Items] |  |  |  |
Contract Term (Years) |  | 20 years |  |
Manitoba Hydro [Member] | Manitoba Hydro PPA Beginning June 2015 (expires May 2020) [Member] | Minnesota Power [Member] |  |  |  |
Power Purchase Agreements [Line Items] |  |  |  |
Output Being Purchased - Electric Energy and Capacity (MW) |  | 50 |  |
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 PPA [Member] | Minnesota Power [Member] | Wind Turbine Generators [Member] | Resource Package [Member] | MPUC [Member] |  |  |  |
Power Purchase Agreements [Line Items] |  |  |  |
Generating Capacity Counterparty Owned (MW) | [1] | 250 |  |
Contract Term (Years) |  | 20 years |  |
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 |  |
TransAlta [Member] | TransAlta Off-Peak Hours PPA (expires 2019) [Member] | Minnesota Power [Member] |  |  |  |
Power Purchase Agreements [Line Items] |  |  |  |
Output Being Purchased - Electric Energy (MW) |  | 50 |  |
TransAlta [Member] | TransAlta On-Peak Hours PPA (expire 2019) [Member] | Minnesota Power [Member] |  |  |  |
Power Purchase Agreements [Line Items] |  |  |  |
Output Being Purchased - Electric Energy (MW) |  | 100 |  |
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%
Minnkota Power [Member] | Minnkota Power PPA (expires May 2020) [Member] | Minnesota Power [Member] |  |  |  |
Power Purchase Agreements [Line Items] |  |  |  |
Output Being Purchased - Electric Energy and Capacity (MW) |  | 50 |  |
Silver Bay Power [Member] | Silver Bay Power Self-Generation [Member] | Minnesota Power [Member] |  |  |  |
Power Purchase Agreements [Line Items] |  |  |  |
Generating Capacity Owned (MW) |  | 90 |  |
Silver Bay Power [Member] | Silver Bay Power Sales Agreement through 2031 (Years 2016-2019) [Member] | Minnesota Power [Member] | Minimum [Member] |  |  |  |
Power Purchase Agreements [Line Items] |  |  |  |
Output Being Sold - Electric Energy (MW) |  | 50 |  |
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 Capacity Only PPA Beginning 2016 (expires 2020) [Member] | Minnesota Power [Member] |  |  |  |
Power Purchase Agreements [Line Items] |  |  |  |
Output Being Purchased - Electric Capacity (MW) |  | 50 |  |
Great River Energy [Member] | Great River Energy Capacity Only PPA Beginning 2017 (expires 2020) [Member] | Minnesota Power [Member] |  |  |  |
Power Purchase Agreements [Line Items] |  |  |  |
Output Being Purchased - Electric Capacity (MW) |  | 50 |  |
Great River Energy [Member] | Great River Energy Capacity and Energy PPA (expires May 2020) [Member] | Minnesota Power [Member] |  |  |  |
Power Purchase Agreements [Line Items] |  |  |  |
Output Being Purchased - Electric Energy and Capacity (MW) |  | 50 |  |
Shell Energy [Member] | Shell Energy PPA [Member] | Minnesota Power [Member] |  |  |  |
Power Purchase Agreements [Line Items] |  |  |  |
Output Being Purchased - Electric Energy (MW) |  | 50 |  |
Basin [Member] | Basin Power Sales Agreement (expires 2019) [Member] | Minnesota Power [Member] |  |  |  |
Power Purchase Agreements [Line Items] |  |  |  |
Output Being Sold Electric - Capacity (MW) |  | 50 |  |
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 |  |
Basin [Member] | Basin PSA (expires April 2020) [Member] | Minnesota Power [Member] |  |  |  |
Power Purchase Agreements [Line Items] |  |  |  |
Output Being Sold Electric - Energy and 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 |  |
[1] | The agreement includes a fixed capacity charge and energy prices that escalate at a fixed rate annually over the term.

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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 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=6499423&loc=d3e659339-123030 + Details
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