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Regulatory Matters - Integrated Resource Plan (Details) - Natural Gas Processing Plant [Member] | Jun. 30, 2015 MW
--- | ---
Minimum [Member] |
Generation [Line Items] |
Generating Capacity (MW) | 200
Maximum [Member] |
Generation [Line Items] |
Generating Capacity (MW) | 300

X
- Definition Capacity of a generating facility, unit, or project in megawatts. + References No definition available. + Details
Name: | ale_GeneratingCapacityMw
Namespace Prefix: | ale_
Data Type: | num:powerItemType
Balance Type: | na
Period Type: | instant

X
- Definition [Line Items] for Generation [Table] + References No definition available. + Details
Name: | ale_GenerationLineItems
Namespace Prefix: | ale_
Data Type: | xbrli:stringItemType
Balance Type: | na
Period Type: | duration

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

X
- Details
Name: | ale_GenerationTypeAxis=us-gaap_NaturalGasProcessingPlantMember
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: |