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Commitments, Guarantees and Contingencies - Other Matters (Details) - USD ($) $ in Millions | 3 Months Ended |
--- | --- | ---
Mar. 31, 2019 | Dec. 31, 2018
ALLETE Clean Energy [Member] | Letters of Credit [Member] |  |
Guarantor Obligations [Line Items] |  |
Collateral | $ 21.6 |
BNI Energy Reclamation Liability [Member] |  |
Guarantor Obligations [Line Items] |  |
Estimated Obligation | 47.5 |
BNI Energy Reclamation Liability [Member] | Letters of Credit [Member] |  |
Guarantor Obligations [Line Items] |  |
Collateral | 0.6 |
BNI Energy Reclamation Liability [Member] | Surety Bonds [Member] |  |
Guarantor Obligations [Line Items] |  |
Collateral | 49.9 |
ALLETE Properties Development and Maintenance Obligations [Member] |  |
Guarantor Obligations [Line Items] |  |
Estimated Obligation | 6.1 |
ALLETE Properties Development and Maintenance Obligations [Member] | Surety Bonds and Letters of Credit [Member] |  |
Guarantor Obligations [Line Items] |  |
Collateral | $ 8.6 |
Town Center District Capital Improvement Bonds [Member] |  |
Guarantor Obligations [Line Items] |  |
Ownership Percentage of Benefited Property | 68.00% | 68.00%
Annual Assessment | $ 1.4 |
Palm Coast Park District Special Assessment Bonds [Member] |  |
Guarantor Obligations [Line Items] |  |
Ownership Percentage of Benefited Property | 12.00% | 19.00%
Annual Assessment | $ 0.6 |

X
- Definition Amount of annual assessments based on ownership percentage of benefited property related to real estate development projects funded by municipal capital improvement revenue and special assessment bonds. + References No definition available. + Details
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Namespace Prefix: | ale_
Data Type: | xbrli:monetaryItemType
Balance Type: | debit
Period Type: | duration

X
- Definition Ownership percentage of property benefitied by a real estate development project funded by municipal capital improvement revenue and special assessment bonds. + References No definition available. + Details
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Namespace Prefix: | ale_
Data Type: | num:percentItemType
Balance Type: | na
Period Type: | instant

X
- Definition Amount of any assets held either as collateral or by third parties that, upon the occurence of any triggering event or condition under the guarantee, can be obtained and liquidated to recover all or a portion of the amounts paid under the guarantee. + References No definition available. + Details
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Namespace Prefix: | ale_
Data Type: | xbrli:monetaryItemType
Balance Type: | credit
Period Type: | instant

X
- 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

X
- Definition Maximum potential amount of future payments (undiscounted) the guarantor could be required to make under the guarantee or each group of similar guarantees before reduction for potential recoveries under recourse or collateralization provisions. + References Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef -Publisher FASB -Name Accounting Standards Codification -Topic 460 -SubTopic 10 -Section 50 -Paragraph 4 -Subparagraph (b)(1) -URI http://asc.fasb.org/extlink&oid=115931803&loc=d3e12069-110248 + Details
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Namespace Prefix: | us-gaap_
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Balance Type: | credit
Period Type: | instant

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

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

X
- Details
Name: | ale_GuaranteeEntityandTypeAxis=ale_BniCoalReclamationLiabilityMember
Namespace Prefix: |
Data Type: | na
Balance Type: |
Period Type: |

X
- Details
Name: | us-gaap_GuaranteeObligationsByNatureAxis=us-gaap_SuretyBondMember
Namespace Prefix: |
Data Type: | na
Balance Type: |
Period Type: |

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

X
- Details
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Namespace Prefix: |
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Balance Type: |
Period Type: |

X
- Details
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Namespace Prefix: |
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Balance Type: |
Period Type: |

X
- Details
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Namespace Prefix: |
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Balance Type: |
Period Type: |