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Commitments, Guarantees and Contingencies - Transmission (Details) $ in Millions | Dec. 31, 2015 USD ($) Miles kV
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CapX2020 [Member] |
Transmission [Line Items] |
Number of Projects | 3
Total Project Costs | $ 100
Great Northern Transmission Line [Member] |
Transmission [Line Items] |
Transmission Line Length (Miles) | Miles | 220
Transmission Line Capacity (kV) | kV | 500
Great Northern Transmission Line [Member] | Minimum [Member] |
Transmission [Line Items] |
Total Project Cost | $ 560
Great Northern Transmission Line [Member] | Maximum [Member] |
Transmission [Line Items] |
Total Project Cost | $ 710
Completed Project - Prior Years [Member] | CapX2020 [Member] |
Transmission [Line Items] |
Number of Projects | 2

X
- Definition Capital cost spent to date for project. + References No definition available. + Details
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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
- 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_
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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_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: |

X
- Details
Name: | us-gaap_RangeAxis=us-gaap_MinimumMember
Namespace Prefix: |
Data Type: | na
Balance Type: |
Period Type: |

X
- Details
Name: | us-gaap_RangeAxis=us-gaap_MaximumMember
Namespace Prefix: |
Data Type: | na
Balance Type: |
Period Type: |

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