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Commitments, Guarantees and Contingencies - Other Matters (Details) (USD $) In Millions, unless otherwise specified | 9 Months Ended |
--- | --- | ---
Sep. 30, 2014 Years | Dec. 31, 2013
BNI Coal Reclamation Liability [Member] |  |
Guarantor Obligations [Line Items] |  |
Surety Bond Amount | $ 49.5 |
Reclamation Letter of Credit | 0.6 |
Total Current Estimated Reclamation Liability | 49.3 |
ALLETE Properties Development and Maintenance Obligations [Member] |  |
Guarantor Obligations [Line Items] |  |
Surety Bond and Letters of Credit | 10.2 |
Development and Maintenance Obligations | 7.4 |
Town Center Community Development District [Member] |  |
Guarantor Obligations [Line Items] |  |
Bond Amount | 26.4 |
Bond Interest Rate | 6.00% |
Bond Term (Years) | 31 |
Ownership of Property Benefited | 72.00% | 73.00%
Annual Assessment | 1.4 |
Palm Coast Park Community Development District [Member] |  |
Guarantor Obligations [Line Items] |  |
Bond Amount | 31.8 |
Bond Interest Rate | 5.70% |
Bond Term (Years) | 31 |
Ownership of Property Benefited | 93.00% | 93.00%
Annual Assessment | $ 2.1 |

X
- Definition Amount of annual assessments based on ownership percentage of benefited property related to real estate development projects funded by municipal capital improvement revenue and special assessment bonds. + References No definition available. + Details
Name: | ale_CommunityDevelopmentDistrictObligationAnnualAssessment
Namespace Prefix: | ale_
Data Type: | xbrli:monetaryItemType
Balance Type: | debit
Period Type: | duration

X
- Definition Interest rate of capital improvement revenue and special assessment bonds used to fund real estate development projects. + References No definition available. + Details
Name: | ale_CommunityDevelopmentDistrictObligationBondInterestRate
Namespace Prefix: | ale_
Data Type: | num:percentItemType
Balance Type: | na
Period Type: | instant

X
- Definition Term (in years) of capital improvement revenue and special assessment bonds used to fund real estate development projects. + References No definition available. + Details
Name: | ale_CommunityDevelopmentDistrictObligationBondTermInYears
Namespace Prefix: | ale_
Data Type: | xbrli:integerItemType
Balance Type: | na
Period Type: | instant

X
- Definition Ownership percentage of property benefitied by a real estate development project funded by municipal capital improvement revenue and special assessment bonds. + References No definition available. + Details
Name: | ale_CommunityDevelopmentDistrictObligationOwnershipOfPropertyBenefited
Namespace Prefix: | ale_
Data Type: | num:percentItemType
Balance Type: | na
Period Type: | instant

X
- Definition Amount of letters of credit that are available for the future payment of environmental commitments and/or contingencies. + References No definition available. + Details
Name: | ale_EnvironmentalLossContingencyLettersOfCredit
Namespace Prefix: | ale_
Data Type: | xbrli:monetaryItemType
Balance Type: | credit
Period Type: | instant

X
- Definition Amount of surety bonds that are available for future payment obligations related to environmental commitments and/or contingencies. + References No definition available. + Details
Name: | ale_EnvironmentalLossContingencySuretyBond
Namespace Prefix: | ale_
Data Type: | xbrli:monetaryItemType
Balance Type: | credit
Period Type: | instant

X
- Definition 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. + References No definition available. + Details
Name: | ale_EnvironmentalLossContingencySuretyBondandLettersofCredit
Namespace Prefix: | ale_
Data Type: | xbrli:monetaryItemType
Balance Type: | credit
Period Type: | instant

X
- Definition Estimated costs remaining on performance and maintenance obligations. + References No definition available. + Details
Name: | ale_PerformanceAndMaintenanceObligations
Namespace Prefix: | ale_
Data Type: | xbrli:monetaryItemType
Balance Type: | credit
Period Type: | instant

X
- Details
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Balance Type: | na
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X
- Definition 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. + References 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 + Details
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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 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 + Details
Name: | us-gaap_SpecialAssessmentBond
Namespace Prefix: | us-gaap_
Data Type: | xbrli:monetaryItemType
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Period Type: | instant