Jointly-Owned Facilities (Details) (USD $) In Millions, unless otherwise specified	Dec. 31, 2012
Jointly-Owned Facilities [Line Items]
Plant in Service	$ 435.9
Accumulated Depreciation	188.5
Construction Work in Progress	50.4
Boswell Unit 4 [Member]
Jointly-Owned Facilities [Line Items]
Plant in Service	413.1
Accumulated Depreciation	188.1
Construction Work in Progress	25.0
% Ownership	80.00%
Generating Unit Capacity (MW)	585
% Not Owned	20.00%
CapX2020 [Member]
Jointly-Owned Facilities [Line Items]
Plant in Service	22.8
Accumulated Depreciation	0.4
Construction Work in Progress	$ 25.4
Number of Projects	3
CapX2020 [Member] \| Minimum [Member]
Jointly-Owned Facilities [Line Items]
% Ownership	9.30%
CapX2020 [Member] \| Maximum [Member]
Jointly-Owned Facilities [Line Items]
% Ownership	14.70%

Capacity of each generating unit/individual project in megawatts.

No definition available.

Name:	ale_GeneratingUnitCapacityMw
Namespace Prefix:	ale_
Data Type:	num:powerItemType
Balance Type:	na
Period Type:	instant

- Definition

Jointly-Owned Electric Facility Percentage Not Owned

+ References

No definition available.

+ Details

Name:	ale_JointlyOwnedElectricFacilityPercentageNotOwned
Namespace Prefix:	ale_
Data Type:	num:percentItemType
Balance Type:	na
Period Type:	instant

- Definition

Number of projects.

+ References

No definition available.

+ Details

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

- Definition

The period end amount owned by the reporting utility of a jointly owned utility plant in service.

+ References

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

-Publisher FASB

-Name Accounting Standards Codification

-Topic 980

-SubTopic 360

-Section S99

-Paragraph 1

-Subparagraph (SAB TOPIC 10.C)

-URI http://asc.fasb.org/extlink&oid=6500719&loc=d3e659983-123033

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

-Publisher SEC

-Name Staff Accounting Bulletin (SAB)

-Number Topic 10

-Section C

+ Details

Name:	us-gaap_JointlyOwnedUtilityPlantGrossOwnershipAmountOfPlantInService
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	instant

- Details

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

- Definition

The period end amount of construction work in progress at a jointly owned electricity generation plant.

+ References

+ Details

Name:	us-gaap_JointlyOwnedUtilityPlantOwnershipAmountOfConstructionWorkInProgress
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	instant

- Definition

The period end amount of accumulated depreciation in a jointly owned electricity generation plant.

+ References

+ Details

Name:	us-gaap_JointlyOwnedUtilityPlantOwnershipAmountOfPlantAccumulatedDepreciation
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	instant

- Definition

The proportionate share of interest (percent) in a jointly owned utility plant.

+ References

+ Details

Name:	us-gaap_JointlyOwnedUtilityPlantProportionateOwnershipShare
Namespace Prefix:	us-gaap_
Data Type:	num:percentItemType
Balance Type:	na
Period Type:	instant