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Investment in ATC (Details) (USD $) In Millions, unless otherwise specified | 3 Months Ended | 9 Months Ended
--- | --- | ---
Sep. 30, 2011 Percent | Sep. 30, 2010 | Sep. 30, 2011 Percent | Sep. 30, 2010
Equity Method Investments and Joint Ventures [Abstract] |  |  |  |
Ownership Percentage | 8.00% |  | 8.00% |
Approved Return on Common Equity | 12.20% |  | 12.20% |
Year-to-Date Investment |  |  | $ 2.0 |
ALLETE's Investment in ATC [Roll Forward] |  |  |  |
Equity Investment Balance as of December 31, 2010 |  |  | 93.3 |
Cash Investments |  |  | 2.0 | 1.2
Equity in ATC Earnings | 4.7 | 4.5 | 13.7 | 13.4
Distributed ATC Earnings |  |  | (11.1) |
Equity Investment Balance as of September 30, 2011 | 97.9 |  | 97.9 |
ATC Summarized Financial Data [Abstract] |  |  |  |
Revenue | 142.8 | 136.9 | 420.6 | 414.1
Operating Expense | 66.4 | 60.2 | 192.5 | 185.9
Other Expense | 19.7 | 21.7 | 61.6 | 64.0
Net Income | 56.7 | 55.0 | 166.5 | 164.2
Equity in ATC Earnings | $ 4.7 | $ 4.5 | $ 13.7 | $ 13.4

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