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Commitments, Guarantees and Contingencies - Environmental Matters (Details) (Boswell Unit 4 [Member], USD $) In Millions, unless otherwise specified	Mar. 31, 2013
Minimum [Member]
Schedule of Utility Generating Facilities [Line Items]
Estimated Capital Expenditures	$ 350
Maximum [Member]
Schedule of Utility Generating Facilities [Line Items]
Estimated Capital Expenditures	$ 400

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
Balance Type:	debit
Period Type:	instant

X

- Details

Name:	ale_ScheduleOfUtilityGeneratingFacilitiesLineItems
Namespace Prefix:	ale_
Data Type:	xbrli:stringItemType
Balance Type:	na
Period Type:	duration