Regulatory Matters - Integrated Resource Plan (Details) $ in Millions	12 Months Ended
Dec. 31, 2020 USD ($) MW
ALLETE, Inc. [Member] \| Nemadji Trail Energy Center [Member]
Regulatory Matters [Line Items]
Estimated Capital Expenditures, Including Past Expenditures \| $	$ 350
Maximum [Member] \| Nemadji Trail Energy Center [Member]
Regulatory Matters [Line Items]
Estimated Capital Expenditures, Including Past Expenditures \| $	700
Maximum [Member] \| ALLETE, Inc. [Member] \| Nemadji Trail Energy Center [Member]
Regulatory Matters [Line Items]
Capital Cost Spent to Date \| $	$ 15
MPUC [Member] \| Minnesota Power [Member] \| Resource Package [Member] \| Natural Gas PPA [Member]
Regulatory Matters [Line Items]
Output Being Purchased (MW)	250
MPUC [Member] \| Natural Gas-Fired [Member] \| Minnesota Power [Member] \| Resource Package [Member] \| Natural Gas PPA [Member]
Regulatory Matters [Line Items]
Expected Output Entitlement (Percent)	50.00%
MPUC [Member] \| Natural Gas-Fired [Member] \| Minimum [Member] \| Resource Package [Member] \| Jointly Owned by ALLETE and Dairyland Power Cooperative [Member] \| Natural Gas PPA [Member] \| Combined-Cycle Natural Gas-Fired Generating Facility [Member] \| Jointly-Owned Electricity Generation Plant [Member]
Regulatory Matters [Line Items]
Generating Capacity Jointly Owned (MW)	525
MPUC [Member] \| Natural Gas-Fired [Member] \| Maximum [Member] \| Resource Package [Member] \| Jointly Owned by ALLETE and Dairyland Power Cooperative [Member] \| Natural Gas PPA [Member] \| Combined-Cycle Natural Gas-Fired Generating Facility [Member] \| Jointly-Owned Electricity Generation Plant [Member]
Regulatory Matters [Line Items]
Generating Capacity Jointly Owned (MW)	550
MPUC [Member] \| Renewable Generation [Member] \| 2021 Integrated Resource Plan [Member]
Regulatory Matters [Line Items]
Generating Capacity (MW)	400

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+ References

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The estimated amount of certain capital expenditures related to environmental compliance, regulatory matters, or other commitments and/or contingencies. This estimate includes expenditures expected in future periods and expenditures made through the end of the current period.

+ References

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Generating Capacity Jointly Owned (MW)

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- Definition

Long-Term Contract for Purchase of Electric Power, Share of Plant Output Purchased

+ References

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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://fasb.org/us-gaap/role/ref/legacyRef
-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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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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