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Investments (Tables) | 3 Months Ended
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Mar. 31, 2018
Investments [Abstract] |
Other Investments [Table Text Block] |
Other Investments | March 31, 2018 |  |  | December 31, 2017 |
Millions |  |  |
ALLETE Properties |  | $26.0 |  |  |  | $26.4 |
Available-for-sale Securities (a) | 21.1 |  |  | 19.1 |
Cash Equivalents | 2.0 |  |  | 3.8 |
Other | 3.7 |  |  | 3.8 |
Total Other Investments |  | $52.8 |  |  |  | $53.1 |
(a) | As of March 31, 2018, the aggregate amount of available-for-sale corporate and governmental debt securities maturing in one year or less was $0.9 million, in one year to less than three years was $3.4 million, in three years to less than five years was $3.3 million and in five or more years was $1.1 million.

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