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Regulatory Matters - Great Northern Transmission Line (GNTL) (Details) - Great Northern Transmission Line [Member] | Sep. 30, 2015 Miles kV
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Transmission [Line Items] |
Transmission Line Length (Miles) | 220
Transmission Line Capacity (kV) | kV | 500

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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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- Definition Line items represent financial concepts included in a table. These concepts are used to disclose reportable information associated with domain members defined in one or many axes to the table. + References No definition available. + Details
Name: | ale_TransmissionLineItems
Namespace Prefix: | ale_
Data Type: | xbrli:stringItemType
Balance Type: | na
Period Type: | duration

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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

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