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Commitments, Guarantees and Contingencies - Transmission (Details) (USD $) In Millions, unless otherwise specified | Mar. 31, 2013 | Dec. 31, 2012
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CapX2020 [Member] |  |
Transmission [Line Items] |  |
Number of CapX2020 Projects | 3 |
Capital Cost Spent to Date | $ 53.5 | $ 48.2
CapX2020 [Member] | Minimum [Member] |  |
Transmission [Line Items] |  |
Estimated Capital Expenditures | 100 |
CapX2020 [Member] | Maximum [Member] |  |
Transmission [Line Items] |  |
Estimated Capital Expenditures | 110 |
CapX2020 Fargo to Monticello Projects [Member] |  |
Transmission [Line Items] |  |
Transmission Line Length (Miles) | 238 |
Transmission Line Capacity (kV) | 345 |
Capital Cost Spent to Date | 42.6 | 37.3
CapX2020 Monticello to St. Cloud Project [Member] |  |
Transmission [Line Items] |  |
Transmission Line Length (Miles) | 28 |
Transmission Line Capacity (kV) | 345 |
CapX2020 Bemidji to Grand Rapids Project [Member] |  |
Transmission [Line Items] |  |
Transmission Line Length (Miles) | 70 |
Transmission Line Capacity (kV) | 230 |
Capital Cost Spent to Date | $ 10.9 | $ 10.9

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- Definition Capital cost spent to date for project. + References No definition available. + Details
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Namespace Prefix: | ale_
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Balance Type: | debit
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X
- Definition The estimated amount of certain future capital expenditures related to environmental compliance, regulatory matters, or other committments and/or contingencies. + References No definition available. + Details
Name: | ale_EstimatedCapitalExpenditures
Namespace Prefix: | ale_
Data Type: | xbrli:monetaryItemType
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- Definition Number of projects. + References No definition available. + Details
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- 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

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- Details
Name: | ale_TransmissionLineItems
Namespace Prefix: | ale_
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- 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