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Regulatory Matters - Integrated Resource Plan (Details) - Natural Gas-Fired [Member]	Sep. 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: