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Operations and Significant Accounting Policies - Business Segments (Details) | 12 Months Ended
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Dec. 31, 2015 a Customers 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 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] |
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] |
Business Segments [Line Items] |
Generating Capacity (MW) | MW | 535
Generating Capacity Constructed and Sold (MW) | MW | 107
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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- Definition Share of plant output being purchased under a long-term contract to purchase electricity from a production plant constructed and financed by a public utility district, cooperative, or other organization. + References 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 Reference 2: http://www.xbrl.org/2003/role/presentationRef -Publisher SEC -Name Staff Accounting Bulletin (SAB) -Number Topic 10 -Section D + Details
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