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Commitments, Guarantees and Contingencies - Transmission (Details) (USD $) In Millions, unless otherwise specified | Dec. 31, 2014
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CapX2020 [Member] |
Transmission [Line Items] |
Number of Projects | 3 ale_NumberOfProjects / ale_TransmissionAxis = ale_Capx2020Member
Estimated Capital Expenditures | $ 105 ale_EstimatedCapitalExpendituresIncludingPastExpenditures / ale_TransmissionAxis = ale_Capx2020Member
Capital Cost Spent to Date | 99 ale_CapitalCostSpentToDate / ale_TransmissionAxis = ale_Capx2020Member
Great Northern Transmission Line [Member] |
Transmission [Line Items] |
Transmission Line Length (Miles) | 220 ale_TransmissionLineLengthMiles / ale_TransmissionAxis = ale_GreatNorthernTransmissionLineMember
Transmission Line Capacity (kV) | 500 ale_TransmissionLineCapacityKv / ale_TransmissionAxis = ale_GreatNorthernTransmissionLineMember
Great Northern Transmission Line [Member] | Minimum [Member] |
Transmission [Line Items] |
Estimated Capital Expenditures | 560 ale_EstimatedCapitalExpendituresIncludingPastExpenditures / us-gaap_RangeAxis = us-gaap_MinimumMember / ale_TransmissionAxis = ale_GreatNorthernTransmissionLineMember
Great Northern Transmission Line [Member] | Maximum [Member] |
Transmission [Line Items] |
Estimated Capital Expenditures | $ 710 ale_EstimatedCapitalExpendituresIncludingPastExpenditures / us-gaap_RangeAxis = us-gaap_MaximumMember / ale_TransmissionAxis = ale_GreatNorthernTransmissionLineMember
Project in Progress [Member] | CapX2020 [Member] |
Transmission [Line Items] |
Number of Projects | 1 ale_NumberOfProjects / ale_ProjectStatusAxis = ale_ProjectinProgressMember / ale_TransmissionAxis = ale_Capx2020Member
Completed Project [Member] [Member] | CapX2020 [Member] |
Transmission [Line Items] |
Number of Projects | 2 ale_NumberOfProjects / ale_ProjectStatusAxis = ale_CompletedProjectMemberMember / ale_TransmissionAxis = ale_Capx2020Member

X
- Definition Capital cost spent to date for project. + References No definition available. + Details
Name: | ale_CapitalCostSpentToDate
Namespace Prefix: | ale_
Data Type: | xbrli:monetaryItemType
Balance Type: | debit
Period Type: | instant

X
- Definition The estimated amount of certain capital expenditures related to environmental compliance, regulatory matters, or other commitments and/or contingencies. This estimate includes expenditures expected in future periods and expenditures made through the end of the current period. + References No definition available. + Details
Name: | ale_EstimatedCapitalExpendituresIncludingPastExpenditures
Namespace Prefix: | ale_
Data Type: | xbrli:monetaryItemType
Balance Type: | debit
Period Type: | instant

X
- Definition Number of projects. + References No definition available. + Details
Name: | ale_NumberOfProjects
Namespace Prefix: | ale_
Data Type: | xbrli:integerItemType
Balance Type: | na
Period Type: | instant

X
- Definition Capacity of a transmission line in kilovolts. + References No definition available. + Details
Name: | ale_TransmissionLineCapacityKv
Namespace Prefix: | ale_
Data Type: | xbrli:integerItemType
Balance Type: | na
Period Type: | instant

X
- Details
Name: | ale_TransmissionLineItems
Namespace Prefix: | ale_
Data Type: | xbrli:stringItemType
Balance Type: | na
Period Type: | duration

X
- Definition Length of a transmission line in miles. + References No definition available. + Details
Name: | ale_TransmissionLineLengthMiles
Namespace Prefix: | ale_
Data Type: | xbrli:integerItemType
Balance Type: | na
Period Type: | instant

X
- Details
Name: | ale_ProjectStatusAxis=ale_CompletedProjectMemberMember
Namespace Prefix: |
Data Type: | na
Balance Type: |
Period Type: |

X
- Details
Name: | ale_ProjectStatusAxis=ale_ProjectinProgressMember
Namespace Prefix: |
Data Type: | na
Balance Type: |
Period Type: |

X
- Details
Name: | ale_TransmissionAxis=ale_Capx2020Member
Namespace Prefix: |
Data Type: | na
Balance Type: |
Period Type: |

X
- Details
Name: | ale_TransmissionAxis=ale_GreatNorthernTransmissionLineMember
Namespace Prefix: |
Data Type: | na
Balance Type: |
Period Type: |