Operations and Significant Accounting Policies - Business Segments (Details) $ in Millions		3 Months Ended	12 Months Ended
	Oct. 01, 2021 USD ($)	Dec. 31, 2021 USD ($) a Customers MW	Dec. 31, 2021 USD ($) a Customers MW	Dec. 31, 2020
Business Segments [Line Items]
Number of Reportable Segments			3	3
Square Butte [Member] | Square Butte PPA [Member]
Business Segments [Line Items]
Minnesota Power Output Entitlement (MW) | MW			227.5
Nemadji Trail Energy Center [Member]
Business Segments [Line Items]
Estimated Capital Expenditures, Including Past Expenditures | $		$ 700.0	$ 700.0
Square Butte [Member] | Square Butte [Member] | Square Butte PPA [Member]
Business Segments [Line Items]
Generating Capacity Counterparty Owned (MW) | MW		455	455
Minnesota Power [Member] | Square Butte [Member] | Square Butte [Member] | Square Butte PPA [Member]
Business Segments [Line Items]
Expected Output Entitlement (Percent)			50.00%
Minnesota Power Output Entitlement (Percent)			50.00%
South Shore Energy | Nemadji Trail Energy Center [Member]
Business Segments [Line Items]
Estimated Capital Expenditures, Including Past Expenditures | $		$ 150.0	$ 150.0
Capital Cost Spent to Date | $		$ 5.0	$ 5.0
Regulated Operations [Member] | Minnesota Power [Member] | Retail Customers [Member] | Electric Rates [Member]
Business Segments [Line Items]
Number of Customers		150,000	150,000
Regulated Operations [Member] | Minnesota Power [Member] | Municipal Customers [Member] | Electric Rates [Member]
Business Segments [Line Items]
Number of Customers		15	15
Regulated Operations [Member] | SWL&P [Member] | Retail Customers [Member] | Electric Rates [Member]
Business Segments [Line Items]
Number of Customers		15,000	15,000
Regulated Operations [Member] | SWL&P [Member] | Retail Customers [Member] | Natural Gas [Member]
Business Segments [Line Items]
Number of Customers		13,000	13,000
Regulated Operations [Member] | SWL&P [Member] | Retail Customers [Member] | Water [Member]
Business Segments [Line Items]
Number of Customers		10,000	10,000
ALLETE Clean Energy [Member] | ALLETE Clean Energy [Member]
Business Segments [Line Items]
Energy Generation Owned (MW) | MW		1,300	1,300
Generating Capacity Under Construction (MW) | MW		200	200
U.S. Water Services [Member]
Business Segments [Line Items]
Approximate Proceeds Expected or Received from Divestiture of Business | $			$ 270.0
Corporate and Other [Member]
Business Segments [Line Items]
Land in Minnesota (Acres) | a		4,000	4,000
Corporate and Other [Member] | South Shore Energy | Nemadji Trail Energy Center [Member]
Business Segments [Line Items]
Gain on Sale After Tax | $	$ 8.5		$ 8.5
Approximate Sales Price | $	$ 20.0
Corporate and Other [Member] | BNI Energy [Member]
Business Segments [Line Items]
Number of Customers		2	2
Corporate and Other [Member] | BNI Energy [Member] | Square Butte [Member]
Business Segments [Line Items]
Number of Customers		1	1
Nobles 2 [Member]
Business Segments [Line Items]
Ownership Percentage		49.00%	49.00%
Natural Gas-Fired [Member] | Nemadji Trail Energy Center [Member]
Business Segments [Line Items]
Generating Capacity Jointly Owned (MW) | MW		600	600
Natural Gas-Fired [Member] | Basin Electric Power Cooperative [Member] | Nemadji Trail Energy Center [Member]
Business Segments [Line Items]
Expected Output Entitlement (Percent)	30.00%
Minnesota Power Output Entitlement (Percent)	30.00%
Natural Gas-Fired [Member] | Dairyland Power Cooperative [Member] | Nemadji Trail Energy Center [Member]
Business Segments [Line Items]
Expected Output Entitlement (Percent)	50.00%
Minnesota Power Output Entitlement (Percent)	50.00%
Natural Gas-Fired [Member] | South Shore Energy | Nemadji Trail Energy Center [Member]
Business Segments [Line Items]
Expected Output Entitlement (Percent)	20.00%	20.00%
Minnesota Power Output Entitlement (Percent)	20.00%	20.00%
Wind Turbine Generators [Member] | Minnesota Power [Member] | Tenaska [Member] | Nobles 2 PPA [Member]
Business Segments [Line Items]
Generating Capacity Counterparty Owned (MW) | MW		250	250
Contract Term (Years)			20 years