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Investments (Tables) | 9 Months Ended
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Sep. 30, 2015
Investments [Abstract] |
Other Investments [Table Text Block] |
Other Investments | September 30, 2015 |  |  | December 31, 2014 |
Millions |  |  |
ALLETE Properties |  | $88.3 |  |  |  | $88.2 |
Available-for-sale Securities (a) | 18.5 |  |  | 18.9 |
Cash Equivalents | 2.6 |  |  | 2.9 |
Other | 4.1 |  |  | 4.4 |
Total Other Investments |  | $113.5 |  |  |  | $114.4 |
(a) | As of September 30, 2015, 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 $1.6 million, in three years to less than five years was $3.0 million, and in five or more years was $6.4 million.
Available-for-sale Securities [Table Text Block] |
Available-For-Sale Securities |  |  |  |
Millions |  | Gross Unrealized |
| Cost | Gain | Loss | Fair Value
September 30, 2015 | $20.1 | $0.1 | $1.7 | $18.5
December 31, 2014 | $19.6 | $0.2 | $0.9 | $18.9
|  | Net | Gross Realized
|  | Proceeds | Gain | Loss
Quarter Ended September 30, |  |  |  |
2015 |  | — | — | —
2014 |  | $0.6 | — | —
Nine Months Ended September 30, |  |  |  |
2015 |  | $0.7 | $0.1 | —
2014 |  | $3.3 | $0.2 | —

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- Definition Tabular disclosure of the reconciliation of available-for-sale securities from cost basis to fair value. + References No definition available. + Details
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