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Commitments, Guarantees and Contingencies - Transmission (Details) (USD $) In Millions, unless otherwise specified | Sep. 30, 2014
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CapX2020 [Member] |
Transmission [Line Items] |
Number of CapX2020 Projects | 3
Capital Cost Spent to Date | $ 98
CapX2020 [Member] | Minimum [Member] |
Transmission [Line Items] |
Estimated Capital Expenditures, Including Past Expenditures | 100
CapX2020 [Member] | Maximum [Member] |
Transmission [Line Items] |
Estimated Capital Expenditures, Including Past Expenditures | 110
Great Northern Transmission Line [Member] |
Transmission [Line Items] |
Transmission Line Length (Miles) | 220
Transmission Line Capacity (kV) | 500
Great Northern Transmission Line [Member] | Minimum [Member] |
Transmission [Line Items] |
Estimated Capital Expenditures, Including Past Expenditures | 560
Great Northern Transmission Line [Member] | Maximum [Member] |
Transmission [Line Items] |
Estimated Capital Expenditures, Including Past Expenditures | $ 710
Project in Progress [Member] | CapX2020 [Member] |
Transmission [Line Items] |
Number of CapX2020 Projects | 1
Completed Project [Member] | CapX2020 [Member] |
Transmission [Line Items] |
Number of CapX2020 Projects | 2

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