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Investment in ATC (Details) - USD ($) $ in Millions |  | 3 Months Ended | 9 Months Ended
--- | --- | --- | ---
Oct. 30, 2015 | Sep. 30, 2015 | Sep. 30, 2014 | Sep. 30, 2015 | Sep. 30, 2014
Investment in ATC [Abstract] |  |  |  |  |
Ownership Percentage |  | 8.00% |  | 8.00% |
Cash Investments |  |  |  | $ 1.2 | $ 3.1
Equity Method Investment, Expected Additional Investment |  | $ 0.0 |  | 0.0 |
Subsequent Event [Line Items] |  |  |  |  |
Cash Investments |  |  |  | 1.2 | 3.1
ALLETE's Investment in ATC [Roll Forward] |  |  |  |  |
Equity Investment Balance as of December 31, 2014 |  |  |  | 121.1 |
Cash Investments |  |  |  | 1.2 | 3.1
Equity Earnings in ATC |  | 5.5 | $ 5.3 | 14.1 | 15.6
Distributed ATC Earnings |  |  |  | (10.4) |
Equity Investment Balance as of September 30, 2015 |  | 126.0 |  | 126.0 |
ATC Summarized Financial Data [Abstract] |  |  |  |  |
Revenue |  | 164.5 | 163.6 | 482.0 | 487.0
Operating Expense |  | 78.0 | 76.6 | 238.3 | 229.6
Other Expense |  | 23.1 | 21.4 | 71.7 | 65.1
Net Income |  | 63.4 | 65.6 | 172.0 | 192.3
ALLETE's Equity in Net Income |  | $ 5.5 | $ 5.3 | $ 14.1 | $ 15.6
Subsequent Event [Member] |  |  |  |  |
Investment in ATC [Abstract] |  |  |  |  |
Cash Investments | $ 0.4 |  |  |  |
Subsequent Event [Line Items] |  |  |  |  |
Cash Investments | 0.4 |  |  |  |
ALLETE's Investment in ATC [Roll Forward] |  |  |  |  |
Cash Investments | $ 0.4 |  |  |  |

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