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Investment in ATC (Details) (USD $) In Millions, unless otherwise specified | 3 Months Ended | 9 Months Ended | 3 Months Ended | 0 Months Ended
--- | --- | --- | --- | ---
Sep. 30, 2014 | Sep. 30, 2013 | Sep. 30, 2014 | Sep. 30, 2013 | Sep. 30, 2014 Sensitivity Analysis [Member] | Oct. 30, 2014 Subsequent Event [Member]
Investment in ATC [Abstract] |  |  |  |  |  |
Ownership Percentage | 8.00% |  | 8.00% |  |  |
Approved Return on Common Equity | 12.20% |  | 12.20% |  |  |
Equity Method Investment, Expected Additional Investment | $ 0 |  | $ 0 |  |  |
Subsequent Event [Line Items] |  |  |  |  |  |
Cash Investments |  |  | 3.1 |  |  | 0.8
ALLETE's Investment in ATC [Roll Forward] |  |  |  |  |  |
Equity Investment Balance as of December 31, 2013 |  |  | 114.6 |  |  |
Cash Investments |  |  | 3.1 |  |  | 0.8
Equity Earnings in ATC | 5.3 | 4.9 | 15.6 | 15.1 |  |
Distributed ATC Earnings |  |  | (12.6) |  |  |
Equity Investment Balance as of September 30, 2014 | 120.7 |  | 120.7 |  |  |
ATC Summarized Financial Data [Abstract] |  |  |  |  |  |
Revenue | 163.6 | 160.5 | 487.0 | 464.3 |  |
Operating Expense | 76.6 | 77.6 | 229.6 | 217.2 |  |
Other Expense | 21.4 | 20.2 | 65.1 | 62.6 |  |
Net Income | 65.6 | 62.7 | 192.3 | 184.5 |  |
ALLETE's Equity in Net Income | 5.3 | 4.9 | 15.6 | 15.1 |  |
Equity Method Investment, Proposed Return on Common Equity | 9.15% |  | 9.15% |  |  |
Sensitivity Analysis [Line Items] |  |  |  |  |  |
Basis Point Reduction on Approved Rate of Return on Common Equity |  |  |  |  | 50 |
Effect on Future Earnings, Amount |  |  |  |  | $ 0.5 |

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