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Investments (Details) (USD $) In Millions, unless otherwise specified | 3 Months Ended | 9 Months Ended | 12 Months Ended
--- | --- | --- | ---
Sep. 30, 2014 | Sep. 30, 2014 | Dec. 31, 2013
Investments [Abstract] |  |  |  |  |  |
ALLETE Properties | $ 89.4 |  | $ 89.4 |  | $ 89.9 |
Available-for-sale Securities | 19.0 | [1] | 19.0 | [1] | 17.7 | [1]
Cash Equivalents | 3.7 | [2] | 3.7 | [2] | 34.2 | [2]
Other | 4.3 |  | 4.3 |  | 4.5 |
Total Other Investments | 116.4 |  | 116.4 |  | 146.3 |
Available-for-sale Corporate Debt Securities, Maturities, One Year or Less | 0.2 |  | 0.2 |  |  |
Available-for-sale Corporate Debt Securities, Maturities, More than One Year to Less than Three Years | 2.1 |  | 2.1 |  |  |
Available-for-sale Corporate Debt Securities, Maturities, Three Years to Less than Five Years | 1.3 |  | 1.3 |  |  |
Available-for-sale Corporate Debt Securities, Maturities, Five Years or More | 7.1 |  | 7.1 |  |  |
ALLETE Properties [Rollforward] |  |  |  |  |  |
Land Inventory Beginning Balance |  |  | 85.4 |  | 86.5 |
Cost of Sales |  |  | (1.1) |  | (1.5) |
Other |  |  | 0.5 |  | 0.4 |
Land Inventory Ending Balance | 84.8 |  | 84.8 |  | 85.4 |
Long-Term Finance Receivables (net of allowances of $0.6 and $0.6) | 1.4 |  | 1.4 |  | 1.4 |
Other | 3.2 |  | 3.2 |  | 3.1 |
Total Real Estate Assets | 89.4 |  | 89.4 |  | 89.9 |
Impairment of Real Estate | 0 |  | 0 |  | 0 |
Long-Term Finance Receivable, Allowance | $ 0.6 |  | $ 0.6 |  | $ 0.6 |
[1] | As of September 30, 2014, the aggregate amount of available-for-sale corporate debt securities maturing in one year or less was $0.2 million, in one year to less than three years was $2.1 million, in three years to less than five years was $1.3 million, and in five or more years was $7.1 million.
[2] | During the first quarter of 2014, cash included in Other Investments was transferred to Cash and Cash Equivalents.

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- Definition Investments in debt and equity securities which are categorized neither as held-to-maturity nor trading and which are intended to be sold or mature more than one year from the balance sheet date or operating cycle, if longer. Such securities are reported at fair value; unrealized gains (losses) related to Available-for-sale Securities are excluded from earnings and reported in a separate component of shareholders' equity (other comprehensive income), unless the Available-for-sale security is designated as a hedge or is determined to have had an other than temporary decline in fair value below its amortized cost basis. All or a portion of the unrealized holding gain (loss) of an Available-for-sale security that is designated as being hedged in a fair value hedge is recognized in earnings during the period of the hedge, as are other than temporary declines in fair value below the cost basis for investments in equity securities and debt securities that an entity intends to sell or it is more likely than not that it will be required to sell before the recovery of its amortized cost basis. Other than temporary declines in fair value below the cost basis for debt securities categorized as Available-for-sale that an entity does not intend to sell and for which it is not more likely than not that the entity will be required to sell before the recovery of its amortized cost basis are bifurcated into credit losses and losses related to all other factors. Other than temporary declines in fair value below cost basis related to credit losses are recognized in earnings, and losses related to all other factors are recognized in other comprehensive income. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 320 -SubTopic 10 -Section 45 -Paragraph 2 -URI http://asc.fasb.org/extlink&oid=6871852&loc=d3e26626-111562 Reference 2: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Glossary Available-for-Sale Securities -URI http://asc.fasb.org/extlink&oid=6505594 + Details
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