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Operations and Significant Accounting Policies - Investments and Other (Details)	12 Months Ended
	Dec. 31, 2013
Long-term Contract for Purchase of Electric Power [Line Items]
Real Estate Sales Revenue Recognition, Minimum Contract Price Cash Collections Requirement (Percent)	20.00%
Square Butte PPA [Member]
Long-term Contract for Purchase of Electric Power [Line Items]
Output Purchased (MW)	227.5
Square Butte Coal-fired Unit [Member] | Square Butte PPA [Member]
Long-term Contract for Purchase of Electric Power [Line Items]
Output Purchased (Percent)	50.00%

X

- Definition

Long-Term Contract for Purchase of Electric Power, Share of Plant Output Purchased

+ References

No definition available.

+ Details

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X

- Definition

The minimum amount of cash collected on the contract price in order to fully recognize profit on the sale of real estate upon closing (company policy).

+ References

No definition available.

+ Details

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Namespace Prefix:	ale_
Data Type:	num:percentItemType
Balance Type:	na
Period Type:	duration

X

- Details

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

X

- Definition

Share of plant output being purchased under a long-term contract to purchase electricity from a production plant constructed and financed by a public utility district, cooperative, or other organization.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef

-Publisher FASB

-Name Accounting Standards Codification

-Topic 980

-SubTopic 10

-Section S99

-Paragraph 1

-Subparagraph (SAB TOPIC 10.D)

-URI http://asc.fasb.org/extlink&oid=6499423&loc=d3e659339-123030



Reference 2: http://www.xbrl.org/2003/role/presentationRef

-Publisher SEC

-Name Staff Accounting Bulletin (SAB)

-Number Topic 10

-Section D

+ Details

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