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Investment in ATC (Details) (USD $) In Millions, unless otherwise specified | 1 Months Ended | 3 Months Ended | 9 Months Ended
--- | --- | --- | ---
Oct. 31, 2012 | Sep. 30, 2012 | Sep. 30, 2011 | Sep. 30, 2012 | Sep. 30, 2011
Investment in ATC [Abstract] |  |  |  |  |
Ownership Percentage |  | 8.00% |  | 8.00% |
Approved Return on Common Equity |  | 12.20% |  | 12.20% |
Equity Method Investment - Additional Investment for Year |  | $ 0 |  | $ 0 |
ALLETE's Investment in ATC [Roll Forward] |  |  |  |  |
Equity Investment Beginning Balance | 105.5 |  |  | 98.9 |
Cash Investments | 0.8 |  |  | 3.9 |
Equity Earnings in ATC |  | 4.9 | 4.7 | 14.3 | 13.7
Distributed ATC Earnings |  |  |  | (11.6) |
Equity Investment Balance as of September 30, 2012 |  | 105.5 |  | 105.5 |
ATC Summarized Financial Data [Abstract] |  |  |  |  |
Revenue |  | 150.3 | 142.8 | 450.1 | 420.6
Operating Expense |  | 68.8 | 66.4 | 210.1 | 192.5
Other Expense |  | 21.0 | 19.7 | 62.1 | 61.6
Net Income |  | 60.5 | 56.7 | 177.9 | 166.5
ALLETE's Equity in Net Income |  | $ 4.9 | $ 4.7 | $ 14.3 | $ 13.7

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