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Operations and Significant Accounting Policies - Minnesota Land Acres (Details) (Investments and Other [Member])	Dec. 31, 2012
Investments and Other [Member]
Business Segments [Line Items]
Number of acres of land in Minnesota (in acres)	6,100

X

- Definition

Number of acres of land available-for-sale.

+ References

No definition available.

+ Details

Name:	ale_NumberOfAcresOfLandAvailableForSaleInAcres
Namespace Prefix:	ale_
Data Type:	xbrli:integerItemType
Balance Type:	na
Period Type:	instant

X

- Details

Name:	us-gaap_SegmentReportingInformationLineItems
Namespace Prefix:	us-gaap_
Data Type:	xbrli:stringItemType
Balance Type:	na
Period Type:	duration