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Investment in ATC (Details) (USD $) In Millions, unless otherwise specified | 1 Months Ended | 3 Months Ended | 6 Months Ended
--- | --- | --- | ---
Jul. 31, 2012 | Jun. 30, 2012 | Jun. 30, 2011 | Jun. 30, 2012 | Jun. 30, 2011
Investment in ATC [Abstract] |  |  |  |  |
Ownership Percentage |  | 8.00% |  | 8.00% |
Approved Return on Common Equity |  | 12.20% |  | 12.20% |
Equity Method Investment - Anticipated Investment for Year |  | $ 4.7 |  | $ 4.7 |
ALLETE's Investment in ATC [Roll Forward] |  |  |  |  |
Equity Investment Beginning Balance | 102.6 |  |  | 98.9 |
Cash Investments | 1.9 |  |  | 2.0 |
Equity Earnings in ATC |  | 4.8 | 4.6 | 9.4 | 9.0
Distributed ATC Earnings |  |  |  | (7.7) |
Equity Investment Balance as of June 30, 2012 |  | 102.6 |  | 102.6 |
ATC Summarized Financial Data [Abstract] |  |  |  |  |
Revenue |  | 152.2 | 138.2 | 299.8 | 277.8
Operating Expense |  | 71.8 | 63.0 | 141.3 | 126.1
Other Expense |  | 21.1 | 19.6 | 41.1 | 41.9
Net Income |  | 59.3 | 55.6 | 117.4 | 109.8
ALLETE's Equity in Net Income |  | $ 4.8 | $ 4.6 | $ 9.4 | $ 9.0

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