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Commitments, Guarantees and Contingencies - Other Matters (Details) (USD $) In Millions, unless otherwise specified | Sep. 30, 2012 | Dec. 31, 2011
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BNI Coal Reclamation Liability [Member] |  |
Surety Bonds [Abstract] |  |
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] |  |
Surety Bonds [Abstract] |  |
Surety Bonds Outstanding | 10.2 |
Performance and Maintenance Obligations | 7.4 |
Performance and Maintenance Obligations, Land Purchaser Contractual Obligation | 0.6 |
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] |  |
Amount | 26.4 |
Interest Rate | 6.00% |
Ownership Percentage of Benefited Property | 73.00% | 73.00%
Annual Assessments Based on Ownership Percentage | 1.5 |
Palm Coast Park Community Development District [Member] |  |
Capital Improvement Revenue/Special Assessment Bonds [Abstract] |  |
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. + 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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- Definition Term in years of municipal bonds used to fund a development project. + References No definition available. + Details
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- Definition Estimated costs remaining on performance and maintenance obligations. + References No definition available. + Details
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- Definition Performance and Maintenance Obligations, Portion that is the Land Purchaser's Contractual Obligation + References No definition available. + Details
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- Details
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- Definition Amount of surety bonds that are available for future payment obligations. + References No definition available. + Details
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- 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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- Definition Total current estimated liability for performance obligation. + References No definition available. + 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 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
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