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Commitments, Guarantees and Contingencies - Other Matters (Details) (USD $) In Millions, unless otherwise specified | Dec. 31, 2011 | Dec. 31, 2010
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BNI Coal Reclamation Liability [Member] |  |
Guarantor Obligations [Line Items] |  |
Surety Bonds Outstanding | $ 29.8 |
Total amount of letter of credit secured in addition to surety bonds for reclamation liability | 2.6 |
Total current estimated reclamation liability | 32.4 |
ALLETE Properties Performance and Maintenance Obligations [Member] |  |
Guarantor Obligations [Line Items] |  |
Surety Bonds Outstanding | 10.2 |
Performance and Maintenance Obligations | 8.0 |
Town Center Community Development District [Member] |  |
Guarantor Obligations [Line Items] |  |
Capital Improvement Revenue/Special Assessment Bonds, Date Issued | March 2005 |
Special Assessment Bond | 26.4 |
Capital Improvement Revenue/Special Assessment Bonds, Interest Rate | 6.00% |
Capital Improvement Revenue/Special Assessment Bonds, Ownership Percentage of Benefited Property | 73.00% | 69.00%
Capital Improvement Revenue/Special Assessment Bonds, Annual Assessments Based on Ownership Percentage | 1.5 |
Palm Coast Park Community Development District [Member] |  |
Guarantor Obligations [Line Items] |  |
Capital Improvement Revenue/Special Assessment Bonds, Date Issued | May 2006 |
Special Assessment Bond | 31.8 |
Capital Improvement Revenue/Special Assessment Bonds, Interest Rate | 5.70% |
Capital Improvement Revenue/Special Assessment Bonds, Ownership Percentage of Benefited Property | 93.00% | 93.00%
Capital Improvement Revenue/Special Assessment Bonds, Annual Assessments Based on Ownership Percentage | $ 2.2 |
Community Development Districts [Member] |  |
Guarantor Obligations [Line Items] |  |
Capital Improvement Revenue/Special Assessment Bonds, Term (Years) | 31 |

X
- Definition Annual assessments based on ownership percentage of property benefited by a development project funded by municipal bonds. + References No definition available. + Details
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X
- Definition Issue date of municipal bonds used to fund a development project. + References No definition available. + Details
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- Definition Interest rate of municipal bonds used to fund a development project. + References No definition available. + Details
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- Definition Ownership percentage of property benefited by a development project funded by municipal bonds. + References No definition available. + Details
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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. + References No definition available. + Details
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Namespace Prefix: | ale_
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Balance Type: | na
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
- Definition Amount of surety bonds that are available for future payment obligations. + References No definition available. + Details
Name: | ale_SuretyBondsOutstanding
Namespace Prefix: | ale_
Data Type: | xbrli:monetaryItemType
Balance Type: | credit
Period Type: | instant

X
- Definition Amount of letter of credit issued in addition to surety bond to secure performance obligation. + References No definition available. + Details
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Namespace Prefix: | ale_
Data Type: | xbrli:monetaryItemType
Balance Type: | credit
Period Type: | instant

X
- Definition Total current estimated liability for performance obligation. + References No definition available. + Details
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Namespace Prefix: | ale_
Data Type: | xbrli:monetaryItemType
Balance Type: | credit
Period Type: | instant

X
- Details
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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
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