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Investments (Tables) | 12 Months Ended
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Dec. 31, 2015
Investments [Abstract] |
Other Investments [Table Text Block] |
Other Investments |  |
As of December 31 | 2015 |  | 2014 |
Millions |  |
ALLETE Properties (a) |  | $50.1 |  |  | $88.2 |
Available-for-sale Securities (b) | 18.5 |  | 18.9 |
Cash Equivalents | 2.0 |  | 2.9 |
Other | 4.0 |  | 4.4 |
Total Other Investments |  | $74.6 |  |  | $114.4 |
(a) | In 2015, ALLETE Properties recorded a $36.3 million non-cash impairment charge related to its real estate assets. (See Note 1. Operations and Significant Accounting Policies.)
(b) | As of December 31, 2015, the aggregate amount of available-for-sale corporate debt securities maturing in one year or less was none, in one year to less than three years was $1.0 million, in three years to less than five years was $3.2 million, and in five or more years was $6.7 million.
Available-for-sale Securities [Table Text Block] |
Available-For-Sale Securities
Millions |  | Gross Unrealized |
As of December 31 | Cost | Gain | Loss | Fair Value
2015 | $20.0 | — | $1.5 | $18.5
2014 | $19.6 | $0.2 | $0.9 | $18.9
2013 | $18.3 | — | $0.6 | $17.7
|  | Net | Gross Realized
Year Ended December 31 |  | Proceeds | Gain | Loss
2015 |  | $1.7 | $0.1 | —
2014 |  | $3.6 | $0.2 | —
2013 |  | $16.1 | $2.2 | —
Available-for-sale Securities [Table Text Block] |
Available-For-Sale Securities
Millions |  | Gross Unrealized |
As of December 31 | Cost | Gain | Loss | Fair Value
2015 | $20.0 | — | $1.5 | $18.5
2014 | $19.6 | $0.2 | $0.9 | $18.9
2013 | $18.3 | — | $0.6 | $17.7
|  | Net | Gross Realized
Year Ended December 31 |  | Proceeds | Gain | Loss
2015 |  | $1.7 | $0.1 | —
2014 |  | $3.6 | $0.2 | —
2013 |  | $16.1 | $2.2 | —

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