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Commitments, Guarantees and Contingencies - Transmission (Details) (USD $) In Millions, unless otherwise specified | Dec. 31, 2011
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Transmission [Line Items] |
MPUC, Billing Factor, Approval Date | May 2011
MPUC, Updated Billing Factor, Filing Date | June 29, 2011
MPUC, Updated Billing Factor, Expected Approval Date | 2012
CapX2020 Bemidji to Grand Rapids Project [Member] |
Transmission [Line Items] |
Transmission Line Length (Miles) | 70
Transmission Line Capacity (kV) | 230
CapX2020 Monticello to St. Cloud Project [Member] |
Transmission [Line Items] |
Transmission Line Length (Miles) | 28
Transmission Line Capacity (kV) | 345
Project Completion Date | December 2011
MPUC, Route Permit, Approval Date | July 2010
CapX2020 Fargo to St. Cloud Project [Member] |
Transmission [Line Items] |
MPUC, Route Permit, Approval Date | June 10, 2011
CapX2020 Fargo to Monticello Projects [Member] |
Transmission [Line Items] |
Transmission Line Length (Miles) | 238
Transmission Line Capacity (kV) | 345
Projected In Service Date | 2015
CapX2020 [Member] |
Transmission [Line Items] |
Number of Projects | 3
Minimum Amount Expected to be Invested in Transmission Projects | 100
Maximum Amount Expected to be Invested in Transmission Projects | 125
Capital Cost Spent to Date | 27.8

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 Date/expected date an asset will be in service. + References No definition available. + Details
Name: | ale_InServiceDate
Namespace Prefix: | ale_
Data Type: | us-types:dateStringItemType
Balance Type: | na
Period Type: | instant

X
- Definition The maximum amount expected to be invested in transmission line projects. + References No definition available. + Details
Name: | ale_MaximumAmountExpectedToBeInvestedInTransmissionProjects
Namespace Prefix: | ale_
Data Type: | xbrli:monetaryItemType
Balance Type: | debit
Period Type: | instant

X
- Definition The minimum amount expected to be invested in transmission line projects. + References No definition available. + Details
Name: | ale_MinimumAmountExpectedToBeInvestedInTransmissionProjects
Namespace Prefix: | ale_
Data Type: | xbrli:monetaryItemType
Balance Type: | debit
Period Type: | instant

X
- Definition Date/expected date of approval by the Minnesota Public Utilities Commission (MPUC) of billing factor. + References No definition available. + Details
Name: | ale_MpucBillingFactorApprovalDate
Namespace Prefix: | ale_
Data Type: | us-types:dateStringItemType
Balance Type: | na
Period Type: | instant

X
- Definition Date/expected date of approval by the Minnesota Public Utilities Commission (MPUC) of route permit. + References No definition available. + Details
Name: | ale_MpucRoutePermitApprovalDate
Namespace Prefix: | ale_
Data Type: | us-types:dateStringItemType
Balance Type: | na
Period Type: | instant

X
- Definition Date/expected date of approval by the Minnesota Public Utilities Commission of updated billing factor. + References No definition available. + Details
Name: | ale_MpucUpdatedBillingFactorApprovalDate
Namespace Prefix: | ale_
Data Type: | us-types:dateStringItemType
Balance Type: | na
Period Type: | instant

X
- Definition Filing date of petition to the Minnesota Public Utilities Commission (MPUC) seeking updated billing factor approval. + References No definition available. + Details
Name: | ale_MpucUpdatedBillingFactorFilingDate
Namespace Prefix: | ale_
Data Type: | us-types:dateStringItemType
Balance Type: | na
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 Date/expected date of project completion. + References No definition available. + Details
Name: | ale_ProjectCompletionDate
Namespace Prefix: | ale_
Data Type: | us-types:dateStringItemType
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