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Investments (Details) - USD ($) shares in Millions, $ in Millions |  | 3 Months Ended | 9 Months Ended |
--- | --- | --- | --- | ---
Sep. 22, 2016 | Sep. 30, 2016 | Sep. 30, 2016 | Dec. 31, 2015
Investments [Abstract] |  |  |  |  |  |
ALLETE Properties |  | $ 32.9 | [1] | $ 32.9 | [1] | $ 50.1
Available-for-sale Securities |  | 19.4 | [2] | 19.4 | [2] | 18.5
Cash Equivalents |  | 2.0 |  | 2.0 |  | 2.0
Other |  | 3.8 |  | 3.8 |  | 4.0
Total Other Investments |  | 58.1 |  | 58.1 |  | $ 74.6
Real Estate Sale Consideration [Abstract] |  |  |  |  |  |
Total Consideration for Land Inventory | $ 21.0 |  |  |  |  |
Down Payment of ALLETE Common Stock, Shares | 0.1 |  |  |  |  |
Down Payment of ALLETE Common Stock, Value | $ 8.0 |  |  | 8.0 |  |
Available-for-sale Securities Corporate Debt Securities, Maturities [Abstract] |  |  |  |  |  |
One Year or Less |  | 0.2 |  | 0.2 |  |
One Year to Less Than Three Years |  | 2.7 |  | 2.7 |  |
Three Years to Less Than Five Years |  | 5.5 |  | 5.5 |  |
Five or More Years |  | 3.3 |  | 3.3 |  |
Impairment of Land Inventory |  | $ 0.0 |  | $ 0.0 |  |
[1] | On September 22, 2016, ALLETE Properties sold its Ormond Crossings project and Lake Swamp wetland mitigation bank for consideration of approximately $21 million. The consideration included a down payment in the form of 0.1 million shares of ALLETE common stock with a value of $8.0 million, with the remaining purchase price to be paid under the terms of a finance receivable due over a five-year period which bears interest at market rates. The finance receivable is collateralized by the property sold.
[2] | As of September 30, 2016, the aggregate amount of available-for-sale corporate and governmental debt securities maturing in one year or less was $0.2 million, in one year to less than three years was $2.7 million, in three years to less than five years was $5.5 million, and in five or more years was $3.3 million.

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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 -Glossary Available-for-Sale Securities -URI http://asc.fasb.org/extlink&oid=6505594 Reference 2: 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=66010992&loc=d3e26626-111562 + Details
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