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Commitments, Guarantees and Contingencies - Power Purchase Agreements (Details) (USD $) In Millions, except Share data, unless otherwise specified | 3 Months Ended
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Mar. 31, 2012
Power Purchase Agreements [Line Items] |
Number of Wind PPAs | 2
Square Butte PPA [Member] |
Power Purchase Agreements [Line Items] |
PPA Expiration Date | 2026
Generating Unit Capacity (MW) | 455
Share of Plant Output (Percent) | 50.00%
Total Debt Outstanding | 429.8
Annual Debt Service | 44
Minnkota Power Sales Agreement [Member] |
Power Purchase Agreements [Line Items] |
PPA Expiration Date | 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 | April 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 | April 2022
Share of Plant Output (MWh) | 1,000,000
Date of MPUC Approval | March 2011
Manitoba Hydro PPA (expires 2035) [Member] |
Power Purchase Agreements [Line Items] |
PPA Start Date | 2020
Term of PPA (Years) | 15
Transmission Line Capacity (kV) | 500
Share of Plant Output (MW) | 250
Date of MPUC Approval | January 26, 2012
DC Transmission Line from Square Butte [Member] |
Power Purchase Agreements [Line Items] |
Transmission Line Purchase Date | December 2009
Transmission Line Capacity (kV) | 250

X
- Definition 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. + References No definition available. + Details
Name: | ale_LongTermContractForPurchaseOfElectricPowerAnticipatedDateOfNewTransmissionLine
Namespace Prefix: | ale_
Data Type: | us-types:dateStringItemType
Balance Type: | na
Period Type: | duration

X
- Definition Long-Term Contract for Purchase of Electric Power, Date of Contract Expiration, Duration String + References No definition available. + Details
Name: | ale_LongTermContractForPurchaseOfElectricPowerDateOfContractExpirationDurationString
Namespace Prefix: | ale_
Data Type: | us-types:durationStringItemType
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Period Type: | duration

X
- 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. + References No definition available. + Details
Name: | ale_LongTermContractForPurchaseOfElectricPowerDateOfMpucApproval
Namespace Prefix: | ale_
Data Type: | us-types:dateStringItemType
Balance Type: | na
Period Type: | duration

X
- 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. + References No definition available. + Details
Name: | ale_LongTermContractForPurchaseOfElectricPowerGeneratingUnitCapacityMw
Namespace Prefix: | ale_
Data Type: | xbrli:integerItemType
Balance Type: | na
Period Type: | duration

X
- 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. + References No definition available. + Details
Name: | ale_LongTermContractForPurchaseOfElectricPowerNumberOfWindPpas
Namespace Prefix: | ale_
Data Type: | xbrli:integerItemType
Balance Type: | na
Period Type: | duration

X
- 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. + References No definition available. + Details
Name: | ale_LongTermContractForPurchaseOfElectricPowerShareOfPlantOutputBeingPurchasedMw
Namespace Prefix: | ale_
Data Type: | xbrli:integerItemType
Balance Type: | na
Period Type: | duration

X
- 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. + References No definition available. + Details
Name: | ale_LongTermContractForPurchaseOfElectricPowerShareOfPlantOutputBeingPurchasedMwh
Namespace Prefix: | ale_
Data Type: | xbrli:integerItemType
Balance Type: | na
Period Type: | duration

X
- 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. + References No definition available. + Details
Name: | ale_LongTermContractForPurchaseOfElectricPowerStartDateOfContract
Namespace Prefix: | ale_
Data Type: | us-types:dateStringItemType
Balance Type: | na
Period Type: | duration

X
- 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. + References No definition available. + Details
Name: | ale_LongTermContractForPurchaseOfElectricPowerTermOfContractYears
Namespace Prefix: | ale_
Data Type: | xbrli:integerItemType
Balance Type: | na
Period Type: | duration

X
- Definition Capacity of a transmission line in kilovolts. + References No definition available. + Details
Name: | ale_TransmissionLineCapacityKv
Namespace Prefix: | ale_
Data Type: | xbrli:integerItemType
Balance Type: | na
Period Type: | instant

X
- Definition Date of purchase of a transmission line. + References No definition available. + Details
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X
- 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. + 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
Name: | us-gaap_LongTermContractForPurchaseOfElectricPowerAmountOfLongTermDebtOrLeaseObligationOutstanding
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Data Type: | xbrli:monetaryItemType
Balance Type: | credit
Period Type: | duration

X
- 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 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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Balance Type: | debit
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- 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 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
Name: | us-gaap_LongTermContractForPurchaseOfElectricPowerShareOfPlantOutputBeingPurchased
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