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Jointly-Owned Facilities and Projects (Details) (USD $) In Millions, unless otherwise specified | Dec. 31, 2014 | Dec. 31, 2013
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Jointly-Owned Facilities and Projects [Line Items] |  |
Plant in Service | $ 474.6 us-gaap_JointlyOwnedUtilityPlantGrossOwnershipAmountOfPlantInService | $ 438.9 us-gaap_JointlyOwnedUtilityPlantGrossOwnershipAmountOfPlantInService
Accumulated Depreciation | 210.8 us-gaap_JointlyOwnedUtilityPlantOwnershipAmountOfPlantAccumulatedDepreciation | 198.5 us-gaap_JointlyOwnedUtilityPlantOwnershipAmountOfPlantAccumulatedDepreciation
Construction Work in Progress | 212.1 us-gaap_JointlyOwnedUtilityPlantOwnershipAmountOfConstructionWorkInProgress | 129.2 us-gaap_JointlyOwnedUtilityPlantOwnershipAmountOfConstructionWorkInProgress
CapX2020 [Member] |  |
Jointly-Owned Facilities and Projects [Line Items] |  |
Number of Projects | 3 ale_NumberOfProjects / ale_TransmissionAxis = ale_Capx2020Member |
Project in Progress [Member] | CapX2020 [Member] |  |
Jointly-Owned Facilities and Projects [Line Items] |  |
Number of Projects | 1 ale_NumberOfProjects / ale_ProjectStatusAxis = ale_ProjectinProgressMember / ale_TransmissionAxis = ale_Capx2020Member |
Completed Project [Member] [Member] | CapX2020 [Member] |  |
Jointly-Owned Facilities and Projects [Line Items] |  |
Number of Projects | 2 ale_NumberOfProjects / ale_ProjectStatusAxis = ale_CompletedProjectMemberMember / ale_TransmissionAxis = ale_Capx2020Member |
Boswell Unit 4 [Member] |  |
Jointly-Owned Facilities and Projects [Line Items] |  |
% Ownership | 80.00%us-gaap_JointlyOwnedUtilityPlantProportionateOwnershipShare / us-gaap_JointlyOwnedUtilityPlantAxis = ale_BoswellUnit4Member | 80.00%us-gaap_JointlyOwnedUtilityPlantProportionateOwnershipShare / us-gaap_JointlyOwnedUtilityPlantAxis = ale_BoswellUnit4Member
Generating Capacity (MW) | 585.0 ale_GeneratingCapacityMw / us-gaap_JointlyOwnedUtilityPlantAxis = ale_BoswellUnit4Member |
% Not Owned | 20.00%ale_JointlyOwnedElectricFacilityPercentageNotOwned / us-gaap_JointlyOwnedUtilityPlantAxis = ale_BoswellUnit4Member |
Plant in Service | 419.1 us-gaap_JointlyOwnedUtilityPlantGrossOwnershipAmountOfPlantInService / us-gaap_JointlyOwnedUtilityPlantAxis = ale_BoswellUnit4Member | 416.1 us-gaap_JointlyOwnedUtilityPlantGrossOwnershipAmountOfPlantInService / us-gaap_JointlyOwnedUtilityPlantAxis = ale_BoswellUnit4Member
Accumulated Depreciation | 209.1 us-gaap_JointlyOwnedUtilityPlantOwnershipAmountOfPlantAccumulatedDepreciation / us-gaap_JointlyOwnedUtilityPlantAxis = ale_BoswellUnit4Member | 197.5 us-gaap_JointlyOwnedUtilityPlantOwnershipAmountOfPlantAccumulatedDepreciation / us-gaap_JointlyOwnedUtilityPlantAxis = ale_BoswellUnit4Member
Construction Work in Progress | 168.1 us-gaap_JointlyOwnedUtilityPlantOwnershipAmountOfConstructionWorkInProgress / us-gaap_JointlyOwnedUtilityPlantAxis = ale_BoswellUnit4Member | 71.5 us-gaap_JointlyOwnedUtilityPlantOwnershipAmountOfConstructionWorkInProgress / us-gaap_JointlyOwnedUtilityPlantAxis = ale_BoswellUnit4Member
CapX2020 Projects [Member] |  |
Jointly-Owned Facilities and Projects [Line Items] |  |
Plant in Service | 55.5 us-gaap_JointlyOwnedUtilityPlantGrossOwnershipAmountOfPlantInService / us-gaap_JointlyOwnedUtilityPlantAxis = ale_Capx2020Member | 22.8 us-gaap_JointlyOwnedUtilityPlantGrossOwnershipAmountOfPlantInService / us-gaap_JointlyOwnedUtilityPlantAxis = ale_Capx2020Member
Accumulated Depreciation | 1.7 us-gaap_JointlyOwnedUtilityPlantOwnershipAmountOfPlantAccumulatedDepreciation / us-gaap_JointlyOwnedUtilityPlantAxis = ale_Capx2020Member | 1.0 us-gaap_JointlyOwnedUtilityPlantOwnershipAmountOfPlantAccumulatedDepreciation / us-gaap_JointlyOwnedUtilityPlantAxis = ale_Capx2020Member
Construction Work in Progress | $ 44.0 us-gaap_JointlyOwnedUtilityPlantOwnershipAmountOfConstructionWorkInProgress / us-gaap_JointlyOwnedUtilityPlantAxis = ale_Capx2020Member | $ 57.7 us-gaap_JointlyOwnedUtilityPlantOwnershipAmountOfConstructionWorkInProgress / us-gaap_JointlyOwnedUtilityPlantAxis = ale_Capx2020Member
CapX2020 Projects [Member] | Minimum [Member] |  |
Jointly-Owned Facilities and Projects [Line Items] |  |
% Ownership | 9.30%us-gaap_JointlyOwnedUtilityPlantProportionateOwnershipShare / us-gaap_JointlyOwnedUtilityPlantAxis = ale_Capx2020Member / us-gaap_RangeAxis = us-gaap_MinimumMember | 9.30%us-gaap_JointlyOwnedUtilityPlantProportionateOwnershipShare / us-gaap_JointlyOwnedUtilityPlantAxis = ale_Capx2020Member / us-gaap_RangeAxis = us-gaap_MinimumMember
CapX2020 Projects [Member] | Maximum [Member] |  |
Jointly-Owned Facilities and Projects [Line Items] |  |
% Ownership | 14.70%us-gaap_JointlyOwnedUtilityPlantProportionateOwnershipShare / us-gaap_JointlyOwnedUtilityPlantAxis = ale_Capx2020Member / us-gaap_RangeAxis = us-gaap_MaximumMember | 14.70%us-gaap_JointlyOwnedUtilityPlantProportionateOwnershipShare / us-gaap_JointlyOwnedUtilityPlantAxis = ale_Capx2020Member / us-gaap_RangeAxis = us-gaap_MaximumMember

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- Definition Capacity of a generating facility, unit, or project in megawatts. + References No definition available. + Details
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Namespace Prefix: | ale_
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Balance Type: | na
Period Type: | instant

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- Definition Jointly-Owned Electric Facility Percentage Not Owned + References No definition available. + Details
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Namespace Prefix: | ale_
Data Type: | num:percentItemType
Balance Type: | na
Period Type: | instant

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- Definition Number of projects. + References No definition available. + Details
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Balance Type: | na
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- Definition The period end amount owned by the reporting utility of a jointly owned utility plant in service. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 980 -SubTopic 360 -Section S99 -Paragraph 1 -Subparagraph (SAB TOPIC 10.C) -URI http://asc.fasb.org/extlink&oid=6500719&loc=d3e659983-123033 Reference 2: http://www.xbrl.org/2003/role/presentationRef -Publisher SEC -Name Staff Accounting Bulletin (SAB) -Number Topic 10 -Section C + Details
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- Details
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X
- Definition The period end amount of construction work in progress at a jointly owned electricity generation plant. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 980 -SubTopic 360 -Section S99 -Paragraph 1 -Subparagraph (SAB TOPIC 10.C) -URI http://asc.fasb.org/extlink&oid=6500719&loc=d3e659983-123033 Reference 2: http://www.xbrl.org/2003/role/presentationRef -Publisher SEC -Name Staff Accounting Bulletin (SAB) -Number Topic 10 -Section C + Details
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- Definition The period end amount of accumulated depreciation in a jointly owned electricity generation plant. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 980 -SubTopic 360 -Section S99 -Paragraph 1 -Subparagraph (SAB TOPIC 10.C) -URI http://asc.fasb.org/extlink&oid=6500719&loc=d3e659983-123033 Reference 2: http://www.xbrl.org/2003/role/presentationRef -Publisher SEC -Name Staff Accounting Bulletin (SAB) -Number Topic 10 -Section C + Details
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Namespace Prefix: | us-gaap_
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Balance Type: | credit
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- Definition The proportionate share of interest (percent) in a jointly owned utility plant. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 980 -SubTopic 360 -Section S99 -Paragraph 1 -Subparagraph (SAB TOPIC 10.C) -URI http://asc.fasb.org/extlink&oid=6500719&loc=d3e659983-123033 Reference 2: http://www.xbrl.org/2003/role/presentationRef -Publisher SEC -Name Staff Accounting Bulletin (SAB) -Number Topic 10 -Section C + Details
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- Details
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Namespace Prefix: |
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- Details
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- Details
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- Details
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Namespace Prefix: |
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- Details
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