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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 |

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- Definition Annual assessments based on ownership percentage of property benefited by a development project funded by municipal bonds. + Details
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- Definition Interest rate of municipal bonds used to fund a development project. + Details
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- Definition Ownership percentage of property benefited by a development project funded by municipal bonds. + Details
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- Definition Term in years of municipal bonds used to fund a development project. + Details
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- Definition Estimated costs remaining on performance and maintenance obligations. + Details
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- Details
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- Definition Amount of surety bonds that are available for future payment obligations. + Details
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- 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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