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Commitments, Guarantees and Contingencies - Other Matters (Details) (USD $) In Millions, unless otherwise specified | 3 Months Ended |
--- | --- | ---
Mar. 31, 2015 Years | Dec. 31, 2014
U.S. Water Services [Member] | Letters of Credit [Member] |  |
Guarantor Obligations [Line Items] |  |
Collateral | $ 3.2 ale_GuarantorObligationsCollateralHeldDirectlyorbyThirdPartiesAmount / ale_GuaranteeEntityandTypeAxis = ale_U.S.WaterServicesMember / us-gaap_GuaranteeObligationsByNatureAxis = us-gaap_FinancialStandbyLetterOfCreditMember |
BNI Coal Reclamation Liability [Member] |  |
Guarantor Obligations [Line Items] |  |
Estimated Obligation | 49.3 us-gaap_GuaranteeObligationsMaximumExposure / ale_GuaranteeEntityandTypeAxis = ale_BniCoalReclamationLiabilityMember |
BNI Coal Reclamation Liability [Member] | Letters of Credit [Member] |  |
Guarantor Obligations [Line Items] |  |
Collateral | 0.6 ale_GuarantorObligationsCollateralHeldDirectlyorbyThirdPartiesAmount / ale_GuaranteeEntityandTypeAxis = ale_BniCoalReclamationLiabilityMember / us-gaap_GuaranteeObligationsByNatureAxis = us-gaap_FinancialStandbyLetterOfCreditMember |
BNI Coal Reclamation Liability [Member] | Surety Bonds [Member] |  |
Guarantor Obligations [Line Items] |  |
Collateral | 49.5 ale_GuarantorObligationsCollateralHeldDirectlyorbyThirdPartiesAmount / ale_GuaranteeEntityandTypeAxis = ale_BniCoalReclamationLiabilityMember / us-gaap_GuaranteeObligationsByNatureAxis = us-gaap_SuretyBondMember |
ALLETE Properties Development and Maintenance Obligations [Member] |  |
Guarantor Obligations [Line Items] |  |
Estimated Obligation | 7.1 us-gaap_GuaranteeObligationsMaximumExposure / ale_GuaranteeEntityandTypeAxis = ale_AlletePropertiesPerformanceAndMaintenanceObligationsMember |
ALLETE Properties Development and Maintenance Obligations [Member] | Surety Bonds and Letters of Credit [Member] |  |
Guarantor Obligations [Line Items] |  |
Collateral | 10.2 ale_GuarantorObligationsCollateralHeldDirectlyorbyThirdPartiesAmount / ale_GuaranteeEntityandTypeAxis = ale_AlletePropertiesPerformanceAndMaintenanceObligationsMember / us-gaap_GuaranteeObligationsByNatureAxis = ale_SuretyBondsandLettersofCreditMember |
Town Center Community Development District Obligation [Member] |  |
Guarantor Obligations [Line Items] |  |
Bond Original Principal | 26.4 ale_CommunityDevelopmentDistrictObligationOriginalPrincipal / ale_GuaranteeEntityandTypeAxis = ale_TownCenterCommunityDevelopmentDistrictMember |
Bond Interest Rate | 6.00%ale_CommunityDevelopmentDistrictObligationBondInterestRate / ale_GuaranteeEntityandTypeAxis = ale_TownCenterCommunityDevelopmentDistrictMember |
Bond Term (in Years) | 31 ale_CommunityDevelopmentDistrictObligationBondTermInYears / ale_GuaranteeEntityandTypeAxis = ale_TownCenterCommunityDevelopmentDistrictMember |
Ownership Percentage of Benefited Property | 72.00%ale_CommunityDevelopmentDistrictObligationOwnershipOfPropertyBenefited / ale_GuaranteeEntityandTypeAxis = ale_TownCenterCommunityDevelopmentDistrictMember | 72.00%ale_CommunityDevelopmentDistrictObligationOwnershipOfPropertyBenefited / ale_GuaranteeEntityandTypeAxis = ale_TownCenterCommunityDevelopmentDistrictMember
Annual Assessment | 1.4 ale_CommunityDevelopmentDistrictObligationAnnualAssessment / ale_GuaranteeEntityandTypeAxis = ale_TownCenterCommunityDevelopmentDistrictMember |
Palm Coast Park Community Development District Obligation [Member] |  |
Guarantor Obligations [Line Items] |  |
Bond Original Principal | 31.8 ale_CommunityDevelopmentDistrictObligationOriginalPrincipal / ale_GuaranteeEntityandTypeAxis = ale_PalmCoastParkCommunityDevelopmentDistrictMember |
Bond Interest Rate | 5.70%ale_CommunityDevelopmentDistrictObligationBondInterestRate / ale_GuaranteeEntityandTypeAxis = ale_PalmCoastParkCommunityDevelopmentDistrictMember |
Bond Term (in Years) | 31 ale_CommunityDevelopmentDistrictObligationBondTermInYears / ale_GuaranteeEntityandTypeAxis = ale_PalmCoastParkCommunityDevelopmentDistrictMember |
Ownership Percentage of Benefited Property | 93.00%ale_CommunityDevelopmentDistrictObligationOwnershipOfPropertyBenefited / ale_GuaranteeEntityandTypeAxis = ale_PalmCoastParkCommunityDevelopmentDistrictMember | 93.00%ale_CommunityDevelopmentDistrictObligationOwnershipOfPropertyBenefited / ale_GuaranteeEntityandTypeAxis = ale_PalmCoastParkCommunityDevelopmentDistrictMember
Annual Assessment | $ 2.1 ale_CommunityDevelopmentDistrictObligationAnnualAssessment / ale_GuaranteeEntityandTypeAxis = ale_PalmCoastParkCommunityDevelopmentDistrictMember |

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
Name: | ale_CommunityDevelopmentDistrictObligationAnnualAssessment
Namespace Prefix: | ale_
Data Type: | xbrli:monetaryItemType
Balance Type: | debit
Period Type: | duration

X
- Definition Interest rate of capital improvement revenue and special assessment bonds used to fund real estate development projects. + References No definition available. + Details
Name: | ale_CommunityDevelopmentDistrictObligationBondInterestRate
Namespace Prefix: | ale_
Data Type: | num:percentItemType
Balance Type: | na
Period Type: | instant

X
- Definition Term (in years) of capital improvement revenue and special assessment bonds used to fund real estate development projects. + References No definition available. + Details
Name: | ale_CommunityDevelopmentDistrictObligationBondTermInYears
Namespace Prefix: | ale_
Data Type: | xbrli:integerItemType
Balance Type: | na
Period Type: | instant

X
- Definition Original principal amount of special assessment bonds used to fund real estate development projects which are not included in Long-term Debt on the Consolidated Balance Sheet. + References No definition available. + Details
Name: | ale_CommunityDevelopmentDistrictObligationOriginalPrincipal
Namespace Prefix: | ale_
Data Type: | xbrli:monetaryItemType
Balance Type: | credit
Period Type: | instant

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
Name: | ale_CommunityDevelopmentDistrictObligationOwnershipOfPropertyBenefited
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
Name: | ale_GuarantorObligationsCollateralHeldDirectlyorbyThirdPartiesAmount
Namespace Prefix: | ale_
Data Type: | xbrli:monetaryItemType
Balance Type: | credit
Period Type: | instant

X
- Details
Name: | us-gaap_GuaranteeObligationsLineItems
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://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 460 -SubTopic 10 -Section 50 -Paragraph 4 -Subparagraph (b)(1) -URI http://asc.fasb.org/extlink&oid=6851643&loc=d3e12069-110248 + Details
Name: | us-gaap_GuaranteeObligationsMaximumExposure
Namespace Prefix: | us-gaap_
Data Type: | xbrli:monetaryItemType
Balance Type: | credit
Period Type: | instant

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

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

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