Operations and Significant Accounting Policies - Business Segments (Details) $ in Millions		12 Months Ended
		Dec. 31, 2019 USD ($) a Customers MW	Dec. 31, 2018
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
Square Butte [Member] | Square Butte [Member] | Square Butte PPA [Member]
Business Segments [Line Items]
Generating Capacity Counterparty Owned (MW) | MW		455
Minnesota Power [Member] | Square Butte [Member] | Square Butte [Member] | Square Butte PPA [Member]
Business Segments [Line Items]
Minnesota Power Output Entitlement (Percent)		50.00%
U.S. Water Services [Member]
Business Segments [Line Items]
Approximate Proceeds from Divestiture of Business | $		$ 270
Regulated Operations [Member] | Minnesota Power [Member] | Retail Customers [Member] | Electric Rates [Member]
Business Segments [Line Items]
Number of Customers		145,000
Regulated Operations [Member] | Minnesota Power [Member] | Municipal Customers [Member] | Electric Rates [Member]
Business Segments [Line Items]
Number of Customers		15
Regulated Operations [Member] | SWL&P [Member] | Retail Customers [Member] | Electric Rates [Member]
Business Segments [Line Items]
Number of Customers		15,000
Regulated Operations [Member] | SWL&P [Member] | Retail Customers [Member] | Natural Gas [Member]
Business Segments [Line Items]
Number of Customers		13,000
Regulated Operations [Member] | SWL&P [Member] | Retail Customers [Member] | Water [Member]
Business Segments [Line Items]
Number of Customers		10,000
ALLETE Clean Energy [Member] | Diamond Spring [Member]
Business Segments [Line Items]
Generating Capacity Owned (MW) | MW		300
ALLETE Clean Energy [Member] | ALLETE Clean Energy [Member]
Business Segments [Line Items]
Generating Capacity Owned (MW) | MW		660
Generating Capacity Under Construction (MW) | MW		380
Corporate and Other [Member]
Business Segments [Line Items]
Land in Minnesota (Acres) | a		4,000
Corporate and Other [Member] | BNI Energy [Domain]
Business Segments [Line Items]
Number of Customers		2
Corporate and Other [Member] | BNI Energy [Domain] | Square Butte [Member]
Business Segments [Line Items]
Number of Customers		1
Nobles 2 [Member]
Business Segments [Line Items]
Ownership Percentage		49.00%
Wind Turbine Generators [Member] | Resource Package [Member] | MPUC [Member] | Minnesota Power [Member] | Tenaska [Member] | Nobles 2 PPA [Member]
Business Segments [Line Items]
Generating Capacity Counterparty Owned (MW) | MW	[1]	250
Contract Term (Years)		20 years

[1] The agreement includes a fixed capacity charge and energy prices that escalate at a fixed rate annually over the term.