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Commitments, Guarantees and Contingencies - Power Purchase Agreements (Details) MWh in Millions, $ in Millions		12 Months Ended
		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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Generating Capacity Counterparty Owned (MW)

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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.

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Long-Term Contract for Purchase of Electric Energy (MW)

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Term in years of long-term contract to purchase electricity from a production plant constructed and financed by a public utility district, cooperative, or other organization.

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Long-term Contract for Sale of Electric Capacity (MW)

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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.

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