Name:	ale_CommunityDevelopmentDistrictObligationAnnualAssessment
Namespace Prefix:	ale_
Data Type:	xbrli:monetaryItemType
Balance Type:	na
Period Type:	duration

Interest rate of capital improvement revenue and special assessment bonds used to fund real estate development projects.

No definition available.

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

Term (in years) of capital improvement revenue and special assessment bonds used to fund real estate development projects.

No definition available.

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

Ownership percentage of property benefitied by a real estate development project funded by municipal capital improvement revenue and special assessment bonds.

No definition available.

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

Amount of letters of credit that are available for the future payment of environmental commitments and/or contingencies.

No definition available.

Name:	ale_EnvironmentalLossContingencyLettersOfCredit
Namespace Prefix:	ale_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	instant

Amount of surety bonds that are available for future payment obligations related to environmental commitments and/or contingencies.

No definition available.

Name:	ale_EnvironmentalLossContingencySuretyBond
Namespace Prefix:	ale_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	instant

Amount of surety bonds outstanding and letters of credit to government entities that are available for future payment obligations related to environmental commitments and contingencies.

No definition available.

Name:	ale_EnvironmentalLossContingencySuretyBondandLettersofCredit
Namespace Prefix:	ale_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	instant

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.

No definition available.

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

Estimated costs remaining on performance and maintenance obligations.

No definition available.

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

- Details

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

Amount of a reclamation and mine closing liability that is associated with a legal obligation for the closure and reclamation of a mine including the removal of buildings, equipment, machinery and other physical remnants of mining, closure of tailings impoundments, leach pads and other mine features, and contouring, covering and revegetation of waste rock piles and other disturbed areas.

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

-Publisher FASB

-Name Accounting Standards Codification

-Topic 450

-SubTopic 20

-Section S99

-Paragraph 1

-Subparagraph (SAB TOPIC 5.Y.Q2)

-URI http://asc.fasb.org/extlink&oid=27011672&loc=d3e149879-122751

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

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.

Reference 1: 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 2: http://www.xbrl.org/2003/role/presentationRef

-Publisher SEC

-Name Regulation S-X (SX)

-Number 210

-Section 02

-Article 5

-Subsection 20, 22

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

-Publisher SEC

-Name Regulation S-X (SX)

-Number 210

-Section 03

-Paragraph 13, 16

-Article 9