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Business Segments (Details) $ in Millions | 3 Months Ended | 6 Months Ended | 12 Months Ended
--- | --- | --- | ---
Jun. 30, 2020 USD ($) a | Jun. 30, 2019 USD ($) | Jun. 30, 2020 USD ($) a Segments | Jun. 30, 2019 USD ($) | Dec. 31, 2019 USD ($)
Business Segments [Line Items] |  |  |  |  |  |  |  |
Number of Reportable Segments | Segments |  |  |  | 3 |  |  |  |
Operating Revenue |  | $ 243.2 | $ 290.4 | $ 554.8 |  | $ 647.6 |  |
Net Income (Loss) Attributable to ALLETE |  | 20.1 | 34.2 | 86.4 |  | 104.7 |  |
Gain on Sale of U.S. Water Services, After-tax |  |  |  |  |  |  |  | $ 13.2
Assets |  | 5,760.4 |  | $ 5,760.4 |  |  |  | 5,482.8
Regulated Operations [Member] |  |  |  |  |  |  |  |
Business Segments [Line Items] |  |  |  |  |  |  |  |
Number of Operating Segments | Segments |  |  |  | 3 |  |  |  |
Operating Revenue |  | 200.8 | 249.8 | $ 466.1 |  | 532.0 |  |
Net Income (Loss) Attributable to ALLETE |  | 11.1 | 30.3 | 68.6 |  | 81.8 |  |
Assets |  | 4,139.8 |  | 4,139.8 |  |  |  | 4,130.8
Regulated Operations [Member] | Residential [Member] |  |  |  |  |  |  |  |
Business Segments [Line Items] |  |  |  |  |  |  |  |
Operating Revenue |  | 27.4 | 29.2 | 68.3 |  | 74.4 |  |
Regulated Operations [Member] | Commercial [Member] |  |  |  |  |  |  |  |
Business Segments [Line Items] |  |  |  |  |  |  |  |
Operating Revenue |  | 28.8 | 34.9 | 66.1 |  | 73.8 |  |
Regulated Operations [Member] | Municipal [Member] |  |  |  |  |  |  |  |
Business Segments [Line Items] |  |  |  |  |  |  |  |
Operating Revenue |  | 9.0 | 12.2 | 19.3 |  | 27.6 |  |
Regulated Operations [Member] | Industrial [Member] |  |  |  |  |  |  |  |
Business Segments [Line Items] |  |  |  |  |  |  |  |
Operating Revenue |  | 88.0 | 120.3 | 206.8 |  | 241.9 |  |
Regulated Operations [Member] | Other Power Suppliers [Member] |  |  |  |  |  |  |  |
Business Segments [Line Items] |  |  |  |  |  |  |  |
Operating Revenue |  | 27.4 | 35.2 | 65.7 |  | 74.6 |  |
Regulated Operations [Member] | Other [Member] |  |  |  |  |  |  |  |
Business Segments [Line Items] |  |  |  |  |  |  |  |
Operating Revenue |  | 20.2 | 18.0 | 39.9 |  | 39.7 |  |
ALLETE Clean Energy [Member] |  |  |  |  |  |  |  |
Business Segments [Line Items] |  |  |  |  |  |  |  |
Operating Revenue |  | 17.6 | 15.4 | 37.7 |  | 32.9 |  |
Net Income (Loss) Attributable to ALLETE |  | 4.0 | 1.9 | 15.7 |  | 7.7 |  |
Assets |  | 1,250.2 |  | 1,250.2 |  |  |  | 1,001.5
ALLETE Clean Energy [Member] | Long-term PSA [Member] |  |  |  |  |  |  |  |
Business Segments [Line Items] |  |  |  |  |  |  |  |
Operating Revenue |  | 14.8 | 12.5 | 32.1 |  | 27.1 |  |
ALLETE Clean Energy [Member] | Other [Member] |  |  |  |  |  |  |  |
Business Segments [Line Items] |  |  |  |  |  |  |  |
Operating Revenue, Other Than Customer Revenue |  | 2.8 | 2.9 | 5.6 |  | 5.8 |  |
U.S. Water Services [Member] |  |  |  |  |  |  |  |
Business Segments [Line Items] |  |  |  |  |  |  |  |
Operating Revenue |  | 0.0 | 0.0 | 0.0 | [1] | 33.4 | [1] |
Net Income (Loss) Attributable to ALLETE | [1] | 0.0 | 0.0 | 0.0 |  | (1.1) |  |
U.S. Water Services [Member] | Point-in-Time [Member] |  |  |  |  |  |  |  |
Business Segments [Line Items] |  |  |  |  |  |  |  |
Operating Revenue | [1] | 0.0 | 0.0 | 0.0 |  | 19.0 |  |
U.S. Water Services [Member] | Contract [Member] |  |  |  |  |  |  |  |
Business Segments [Line Items] |  |  |  |  |  |  |  |
Operating Revenue |  | 0.0 | 0.0 | 0.0 | [1] | 9.2 | [1] |
U.S. Water Services [Member] | Capital Project [Member] |  |  |  |  |  |  |  |
Business Segments [Line Items] |  |  |  |  |  |  |  |
Operating Revenue |  | $ 0.0 | 0.0 | $ 0.0 | [1] | 5.2 | [1] |
Corporate and Other [Member] |  |  |  |  |  |  |  |
Business Segments [Line Items] |  |  |  |  |  |  |  |
Number of Operating Segments | Segments |  |  |  | 2 |  |  |  |
Land in Minnesota (Acres) | a |  | 4,000 |  | 4,000 |  |  |  |
Operating Revenue |  | $ 24.8 | 25.2 | $ 51.0 |  | 49.3 |  |
Net Income (Loss) Attributable to ALLETE | [1] | 5.0 | 2.0 | 2.1 |  | 16.3 |  |
Gain on Sale of U.S. Water Services, After-tax |  |  |  |  |  | 11.1 |  |
Assets |  | 370.4 |  | 370.4 |  |  |  | $ 350.5
Corporate and Other [Member] | Long-term Contract [Member] |  |  |  |  |  |  |  |
Business Segments [Line Items] |  |  |  |  |  |  |  |
Operating Revenue |  | 22.5 | 21.2 | 44.9 |  | 41.4 |  |
Corporate and Other [Member] | Other [Member] |  |  |  |  |  |  |  |
Business Segments [Line Items] |  |  |  |  |  |  |  |
Operating Revenue |  | $ 2.3 | $ 4.0 | $ 6.1 |  | $ 7.9 |  |
[1] | In March 2019, ALLETE sold U.S. Water Services. The Company recognized a gain on the sale of $11.1 million after-tax during the six months ended June 30, 2019, which is reflected in Corporate and Other.

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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_
Data Type: | xbrli:stringItemType
Balance Type: | na
Period Type: | duration

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

X
- Details
Name: | us-gaap_ContractWithCustomerSalesChannelAxis=ale_ResidentialMember
Namespace Prefix: |
Data Type: | na
Balance Type: |
Period Type: |

X
- Details
Name: | us-gaap_ContractWithCustomerSalesChannelAxis=ale_CommercialMember
Namespace Prefix: |
Data Type: | na
Balance Type: |
Period Type: |

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- Details
Name: | us-gaap_ContractWithCustomerSalesChannelAxis=ale_MunicipalMember
Namespace Prefix: |
Data Type: | na
Balance Type: |
Period Type: |

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

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

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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: |

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- Details
Name: | us-gaap_ContractWithCustomerSalesChannelAxis=ale_LongtermPSAMember
Namespace Prefix: |
Data Type: | na
Balance Type: |
Period Type: |

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- Details
Name: | us-gaap_ContractWithCustomerSalesChannelAxis=ale_ALLETECleanEnergyOtherMember
Namespace Prefix: |
Data Type: | na
Balance Type: |
Period Type: |

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- Details
Name: | us-gaap_StatementBusinessSegmentsAxis=ale_U.S.WaterServicesMember
Namespace Prefix: |
Data Type: | na
Balance Type: |
Period Type: |

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

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

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

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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_ContractWithCustomerSalesChannelAxis=ale_LongtermContractMember
Namespace Prefix: |
Data Type: | na
Balance Type: |
Period Type: |

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