Commitments, Guarantees and Contingencies - Power Purchase Agreements (Details) MWh in Millions, $ in Millions	12 Months Ended
	Dec. 31, 2020 USD ($) MWh MW	Dec. 31, 2019 USD ($)	Dec. 31, 2018 USD ($)
Square Butte [Member] | Square Butte PPA (expires 2026) [Member]
Power Purchase Agreements [Line Items]
PPA Counterparty Total Debt Outstanding | $	$ 268.6
Square Butte [Member] | Square Butte PPA (expires 2026) [Member] | Minnesota Power [Member]
Power Purchase Agreements [Line Items]
Cost of Purchased Power | $	79.5	$ 82.7	$ 78.0
Pro Rata Share of PPA Counterparty Interest Expense | $	$ 7.1	$ 8.3	$ 9.1
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 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] | Silver Bay Power Self-Generation [Member] | Minnesota Power [Member]
Power Purchase Agreements [Line Items]
Energy Generation 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 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] | Square Butte Coal-fired Unit [Member] | Minnesota Power [Member]
Power Purchase Agreements [Line Items]
PPA Counterparty Annual Debt Service through 2024 | $	$ 30.5
2021-01-01 [Member] | Square Butte [Member] | Square Butte PPA (expires 2026) [Member] | Square Butte Coal-fired Unit [Member] | Minnesota Power [Member]
Power Purchase Agreements [Line Items]
PPA Counterparty Annual Debt Service through 2024 | $	45.5
2025-01-01 [Member] | Square Butte [Member] | Square Butte PPA (expires 2026) [Member] | Square Butte Coal-fired Unit [Member] | Minnesota Power [Member]
Power Purchase Agreements [Line Items]
PPA Counterparty Annual Debt Service through 2024 | $	$ 26.6