Operations and Significant Accounting Policies - Business Segments (Details)	12 Months Ended
Dec. 31, 2017 a Customers MW
Business Segments [Line Items]
Number of Reportable Segments	3
Square Butte [Member] \| Square Butte PPA [Member]
Business Segments [Line Items]
Minnesota Power Output Entitlement (Percent)	50.00%
Minnesota Power Output Entitlement (MW) \| MW	227.5
Regulated Operations [Member]
Business Segments [Line Items]
Alternative Revenue Program, Required Collection Period (Months)	24 months
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 Owned (MW) \| MW	535
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

Capacity of a generating facility, unit, or project in megawatts.

+ References

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- Definition

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+ References

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- Definition

Number of Customers

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Area of land held.

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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

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Number of segments reported by the entity. A reportable segment is a component of an entity for which there is an accounting requirement to report separate financial information on that component in the entity's financial statements.

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Line items represent financial concepts included in a table. These concepts are used to disclose reportable information associated with domain members defined in one or many axes to the table.

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