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Commitments, Guarantees and Contingencies - Other Matters (Details) (USD $) In Millions, unless otherwise specified	Sep. 30, 2011 Percent	Dec. 31, 2010 Percent
BNI Coal Reclamation Liability [Member]
Surety Bonds [Abstract]
Surety Bonds Outstanding	$ 29.7
ALLETE Properties Performance and Maintenance Obligations [Member]
Surety Bonds [Abstract]
Surety Bonds Outstanding	10.2
Performance and Maintenance Obligations	8.0
Community Development Districts [Member]
Capital Improvement Revenue/Special Assessment Bonds [Abstract]
Term (Years)	31
Town Center Community Development District [Member]
Capital Improvement Revenue/Special Assessment Bonds [Abstract]
Date Issued	March 2005
Amount	26.4
Interest Rate	6.00%
Ownership Percentage of Benefited Property	73.00%	69.00%
Annual Assessments Based on Ownership Percentage	1.5
Palm Coast Park Community Development District [Member]
Capital Improvement Revenue/Special Assessment Bonds [Abstract]
Date Issued	May 2006
Amount	31.8
Interest Rate	5.70%
Ownership Percentage of Benefited Property	93.00%	93.00%
Annual Assessments Based on Ownership Percentage	$ 2.2

X

- Definition

Annual assessments based on ownership percentage of property benefited by a development project funded by municipal bonds.

+ Details

Name:	ale_CapitalImprovementRevenuespecialAssessmentBondsAnnualAssessmentsBasedOnOwnershipPercentage
Namespace Prefix:	ale
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	instant

X

- Definition

Issue date of municipal bonds used to fund a development project.

+ Details

Name:	ale_CapitalImprovementRevenuespecialAssessmentBondsDateIssued
Namespace Prefix:	ale
Data Type:	us-types:dateStringItemType
Balance Type:	na
Period Type:	instant

X

- Definition

Interest rate of municipal bonds used to fund a development project.

+ Details

Name:	ale_CapitalImprovementRevenuespecialAssessmentBondsInterestRate
Namespace Prefix:	ale
Data Type:	num:percentItemType
Balance Type:	na
Period Type:	instant

X

- Definition

Ownership percentage of property benefited by a development project funded by municipal bonds.

+ Details

Name:	ale_CapitalImprovementRevenuespecialAssessmentBondsOwnershipPercentageOfBenefitedProperty
Namespace Prefix:	ale
Data Type:	num:percentItemType
Balance Type:	na
Period Type:	instant

X

- Definition

Term in years of municipal bonds used to fund a development project.

+ Details

Name:	ale_CapitalImprovementRevenuespecialAssessmentBondsTermYears
Namespace Prefix:	ale
Data Type:	xbrli:integerItemType
Balance Type:	na
Period Type:	instant

X

- Definition

Estimated costs remaining on performance and maintenance obligations.

+ Details

Name:	ale_PerformanceAndMaintenanceObligations
Namespace Prefix:	ale
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	instant

X

- Details

Name:	ale_SuretyBondsAbstract
Namespace Prefix:	ale
Data Type:	xbrli:stringItemType
Balance Type:	na
Period Type:	duration

X

- Definition

Amount of surety bonds that are available for future payment obligations.

+ Details

Name:	ale_SuretyBondsOutstanding
Namespace Prefix:	ale
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	instant

X

- Definition

Including the current and noncurrent portions, carrying value as of the balance sheet date of a type of municipal bond typically used to fund a development project. Also called special assessment limited liability bond, special district bond, special purpose bond, special tax bond and community development obligation. Interest owed to lenders is paid by taxes levied on the community benefiting from the particular bond-funded project. For example, if a bond of this sort was issued to pay for sidewalks to be repaved in a certain community, an additional tax would be levied on homeowners in the area benefiting from this project.

+ References

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

-Publisher SEC

-Name Regulation S-X (SX)

-Number 210

-Section 03

-Paragraph 13, 16

-Article 9



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

-Publisher FASB

-Name Accounting Standards Codification

-Topic 944

-SubTopic 210

-Section S99

-Paragraph 1

-Subparagraph (SX 210.7-03.16)

-URI http://asc.fasb.org/extlink&oid=6879938&loc=d3e572229-122910



Reference 3: http://www.xbrl.org/2003/role/presentationRef

-Publisher SEC

-Name Regulation S-X (SX)

-Number 210

-Article 5

-Section 02

-Subsection 20, 22

+ Details

Name:	us-gaap_SpecialAssessmentBond
Namespace Prefix:	us-gaap
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	instant

X

- Details

Name:	us-gaap_SpecialAssessmentBondCurrentAndNoncurrentAbstract
Namespace Prefix:	us-gaap
Data Type:	xbrli:stringItemType
Balance Type:	na
Period Type:	duration