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Investments (Tables) | 12 Months Ended
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Dec. 31, 2018
Investments [Abstract] |
Other Investments [Table Text Block] |
Other Investments |  |
As of December 31 | 2018 |  | 2017 |
Millions |  |
ALLETE Properties |  | $24.4 |  |  | $26.4 |
Available-for-sale Securities (a) | 20.2 |  | 19.1 |
Cash Equivalents | 1.0 |  | 3.8 |
Other | 3.5 |  | 3.8 |
Total Other Investments |  | $49.1 |  |  | $53.1 |
(a) | As of December 31, 2018, the aggregate amount of available-for-sale corporate and governmental debt securities maturing in one year or less was $2.0 million, in one year to less than three years was $2.9 million, in three years to less than five years was $2.6 million, and in five or more years was $0.5 million.

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