Operations and Significant Accounting Policies - Business Segments (Details)	12 Months Ended
	Dec. 31, 2018 a Customers MW	Dec. 31, 2017
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
Minnesota Power [Member] | Square Butte [Member] | Square Butte [Member] | Square Butte PPA [Member]
Business Segments [Line Items]
Minnesota Power Output Entitlement (Percent)	50.00%
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	16
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] | ALLETE Clean Energy [Member]
Business Segments [Line Items]
Generating Capacity Owned (MW) | MW	545
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