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Operations and Significant Accounting Policies - ALLETE Clean Energy Asset Acquisition (Details) - Caddo [Member] - ALLETE Clean Energy [Member] $ in Millions | 6 Months Ended |
--- | --- | ---
Jun. 30, 2020 MW | Mar. 10, 2020 USD ($)
Business Segments [Line Items] |  |
Cash Paid for Assets Acquired | $ |  | $ 8
Generating Capacity Under Development (MW) | MW | 300 |

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- Definition Cash Paid for Assets Acquire + References No definition available. + Details
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Namespace Prefix: | ale_
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Balance Type: | credit
Period Type: | instant

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- Definition Generating Capacity Under Development (MW) + References No definition available. + Details
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Namespace Prefix: | ale_
Data Type: | num:powerItemType
Balance Type: | na
Period Type: | duration

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- Definition 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. + References No definition available. + Details
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Namespace Prefix: | us-gaap_
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Balance Type: | na
Period Type: | duration

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- Details
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Namespace Prefix: |
Data Type: | na
Balance Type: |
Period Type: |

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- Details
Name: | us-gaap_StatementBusinessSegmentsAxis=ale_ALLETECleanEnergyMember
Namespace Prefix: |
Data Type: | na
Balance Type: |
Period Type: |