Regulatory Matters - Nemadji Trail Energy Center (Details) $ in Millions	3 Months Ended
Mar. 31, 2021 USD ($) MW
Nemadji Trail Energy Center [Member] \| Maximum [Member]
Regulatory Matters [Line Items]
Estimated Capital Expenditures, Including Past Expenditures \| $	$ 700
ALLETE, Inc. [Member] \| Nemadji Trail Energy Center [Member]
Regulatory Matters [Line Items]
Estimated Capital Expenditures, Including Past Expenditures \| $	350
ALLETE, Inc. [Member] \| Nemadji Trail Energy Center [Member] \| Maximum [Member]
Regulatory Matters [Line Items]
Total Project Costs Incurred to Date \| $	$ 15
MPUC [Member] \| Resource Package [Member] \| Natural Gas-fired [Member] \| Combined-Cycle Natural Gas-Fired Generating Facility [Member] \| Minimum [Member] \| Jointly Owned by ALLETE and Dairyland Power Cooperative [Member] \| Natural Gas PPA [Member] \| Jointly Owned Electricity Generation Plant [Member]
Regulatory Matters [Line Items]
Generating Capacity to be Jointly Owned (MW) \| MW	525
MPUC [Member] \| Resource Package [Member] \| Natural Gas-fired [Member] \| Combined-Cycle Natural Gas-Fired Generating Facility [Member] \| Maximum [Member] \| Jointly Owned by ALLETE and Dairyland Power Cooperative [Member] \| Natural Gas PPA [Member] \| Jointly Owned Electricity Generation Plant [Member]
Regulatory Matters [Line Items]
Generating Capacity to be Jointly Owned (MW) \| MW	550
MPUC [Member] \| Resource Package [Member] \| Minnesota Power [Member] \| Natural Gas PPA [Member]
Regulatory Matters [Line Items]
Output Being Purchased (MW) \| MW	250
MPUC [Member] \| Resource Package [Member] \| Minnesota Power [Member] \| Natural Gas-fired [Member] \| Natural Gas PPA [Member]
Regulatory Matters [Line Items]
Expected Output Entitlement (Percent)	50.00%

Capital cost spent to date for project.

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

+ References

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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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