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Commitments, Guarantees and Contingencies - Environmental Matters (Details) (Boswell Unit 4 [Member], USD $) In Millions, unless otherwise specified	Dec. 31, 2012
Boswell Unit 4 [Member]
Schedule of Utility Generating Facilities [Line Items]
Estimated Environmental Compliance Costs - Minimum	$ 350
Estimated Environmental Compliance Costs - Maximum	$ 400

X

- Definition

The maximum amount expected to be invested related to compliance with a certain environmental regualtion/mandate.

+ 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 minimum amount expected to be invested related to compliance with a certain environmental regualtion/mandate.

+ References

No definition available.

+ Details

Name:	ale_EstimatedEnvironmentalComplianceCostsMinimum
Namespace Prefix:	ale_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	instant

X

- Details

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