Commitments, Guarantees and Contingencies - Power Purchase Agreements (Details) (USD $) In Millions, except Share data, unless otherwise specified	9 Months Ended
Sep. 30, 2011
Power Purchase Agreements [Line Items]
Number of Wind PPAs	2
DC Transmission Line from Square Butte [Member]
Power Purchase Agreements [Line Items]
Transmission Line Purchase Date	December 2009
Transmission Line Capacity (kV)	250
Square Butte PPA [Member]
Power Purchase Agreements [Line Items]
PPA Expiration Date	Dec. 31, 2026
Generating Unit Capacity (MW)	455
Share of Plant Output (Percent)	50.00%
Total Debt Outstanding	$ 421.6
Annual Debt Service	$ 39
Minnkota Power Sales Agreement [Member]
Power Purchase Agreements [Line Items]
PPA Expiration Date	Dec. 31, 2025
Anticipated Date of New Transmission Line	2013
Oliver Wind I PPA [Member]
Power Purchase Agreements [Line Items]
PPA Start Date	2006
Share of Plant Output (MW)	50
Oliver Wind II PPA [Member]
Power Purchase Agreements [Line Items]
PPA Start Date	2007
Share of Plant Output (MW)	48
Oliver Wind I and II PPA [Member]
Power Purchase Agreements [Line Items]
Term of PPA (Years)	25
Manitoba Hydro PPA (expires April 2015) [Member]
Power Purchase Agreements [Line Items]
PPA Expiration Date	Apr. 30, 2015
Share of Plant Output (MW)	50
Manitoba Hydro PPA (expires April 2022) [Member]
Power Purchase Agreements [Line Items]
PPA Start Date	May 2011
PPA Expiration Date	Apr. 30, 2022
Share of Plant Output (MWh)	1,000,000
Date of MPUC Approval	Mar. 31, 2011

Anticipated date that a new transmission line will be built as part of a long-term contract to purchase electricity from a production plant constructed and financed by a public utility district, cooperative, or other organization.

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- Definition

Date of approval by Minnesota Public Utilities Commission (MPUC) of long-term contract to purchase electricity from a production plant constructed and financed by a public utility district, cooperative, or other organization.

+ Details

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- Definition

The generating capacity of a production plant constructed and financed by a public utility district, cooperative, or other organization with whom exists a long-term contract to purchase electric power.

+ Details

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- Definition

Number of long-term wind contracts to purchase electricity from a production plant constructed and financed by a public utility district, cooperative, or other organization.

+ Details

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- Definition

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

+ Details

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- Definition

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

+ Details

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- Definition

Start date of long-term contract to purchase electricity from a production plant constructed and financed by a public utility district, cooperative, or other organization.

+ Details

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- Definition

Term in years of long-term contract to purchase electricity from a production plant constructed and financed by a public utility district, cooperative, or other organization.

+ Details

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Namespace Prefix:	ale
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Period Type:	duration

- Definition

Capacity of a transmission line in kilovolts.

+ Details

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Data Type:	xbrli:integerItemType
Balance Type:	na
Period Type:	instant

- Definition

Date of purchase of a transmission line.

+ Details

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- Definition

+ References

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

-Publisher SEC

-Name Staff Accounting Bulletin (SAB)

-Number Topic 10

-Section D

Reference 2: 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

+ Details

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Data Type:	xbrli:monetaryItemType
Balance Type:	credit
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- Definition

Under long-term contracts with public utility districts, cooperatives or other organizations, a utility company receives a portion of the output of a production plant constructed and financed by the district or cooperative. The utility has only a nominal or no investment at all in the plant but pays a proportionate part of the plant's costs, including debt service. If the utility is obligated to pay certain minimum amounts which cover debt service requirements whether or not the plant is operating, the amount can be disclosed here.

+ References

+ Details

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- Definition

Date of contract expiration of long-term contract to purchase electricity from a production plant constructed and financed by a public utility district, cooperative, or other organization.

+ References

+ Details

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- Definition

Line items represent financial concepts included in a table. These concepts are used to disclose reportable information associated with domain members defined in one or many axes to the table.

+ Details

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- 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

+ Details

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