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Operations and Significant Accounting Policies - Business Segments (Details)	12 Months Ended
	Dec. 31, 2016 a Customers MW	Dec. 31, 2015 MW
Business Segments [Line Items]
Number of Reportable Segments	3
Square Butte [Member] | Square Butte PPA [Member]
Business Segments [Line Items]
Output Purchased (Percent)	50.00%
Output Being Purchased (MW) | MW	227.5
Regulated Operations [Member] | Minnesota Power [Member] | Retail Customers [Member] | Electric [Member]
Business Segments [Line Items]
Number of Customers	145,000
Regulated Operations [Member] | Minnesota Power [Member] | Municipal Customers [Member] | Electric [Member]
Business Segments [Line Items]
Number of Customers	16
Regulated Operations [Member] | SWL&P [Member] | Retail Customers [Member] | Electric [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 (MW) | MW	535
Generating Capacity Constructed and Sold (MW) | MW		107
ALLETE Clean Energy [Member] | ALLETE Clean Energy [Member] | ALLETE Clean Energy PSA - Future Development Project [Member]
Business Segments [Line Items]
Future Generating Capacity Under Development (MW) | MW	50
PSA Term (Years)	25 years
Corporate and Other [Member]
Business Segments [Line Items]
Land in Minnesota (Acres) | a	5,000
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

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Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 980
-SubTopic 10
-Section S99
-Paragraph 1
-Subparagraph (SAB Topic 10.D)
-URI http://asc.fasb.org/extlink&oid=6499423&loc=d3e659339-123030

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