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Investment in ATC (Details) $ in Millions |  | 3 Months Ended | 6 Months Ended
--- | --- | --- | ---
Jul. 29, 2016 USD ($) | Jun. 30, 2016 USD ($) | Jun. 30, 2015 USD ($) | Jun. 30, 2016 USD ($) | Jun. 30, 2015 USD ($)
ALLETE's Investment in ATC [Roll Forward] |  |  |  |  |
Equity Investment Balance as of December 31, 2015 |  |  |  | $ 124.5 |
Cash Investments |  |  |  | 1.6 | $ 0.8
Equity in ATC Earnings |  | $ 4.1 | $ 4.7 | 8.9 | $ 8.6
Equity Investment Balance as of June 30, 2016 |  | $ 129.0 |  | $ 129.0 |
ATC [Member] |  |  |  |  |
Investment in ATC [Line Items] |  |  |  |  |
Ownership Percentage |  | 8.00% |  | 8.00% |
Expected Additional Investment in 2016 |  | $ 2.7 |  | $ 2.7 |
ALLETE's Investment in ATC [Roll Forward] |  |  |  |  |
Equity Investment Balance as of December 31, 2015 |  |  |  | 124.5 |
Cash Investments |  |  |  | 1.6 |
Equity in ATC Earnings |  |  |  | 8.9 |
Distributed ATC Earnings |  |  |  | (6.0) |
Equity Investment Balance as of June 30, 2016 |  | $ 129.0 |  | $ 129.0 |
Authorized Return on Equity |  | 12.20% |  | 12.20% |
ATC [Member] | Subsequent Event [Member] |  |  |  |  |
ALLETE's Investment in ATC [Roll Forward] |  |  |  |  |
Cash Investments | $ 1.9 |  |  |  |
Sensitivity Analysis [Member] | ATC [Member] |  |  |  |  |
ALLETE's Investment in ATC [Roll Forward] |  |  |  |  |
Basis Point Reduction on Approved Rate of Return on Common Equity |  | 50 |  | 50 |
After-tax [Member] | Sensitivity Analysis [Member] | ATC [Member] |  |  |  |  |
ALLETE's Investment in ATC [Roll Forward] |  |  |  |  |
Annual Effect on Future Equity Earnings in ATC |  |  |  | $ 0.5 |
Pre-tax [Member] | Sensitivity Analysis [Member] | ATC [Member] |  |  |  |  |
ALLETE's Investment in ATC [Roll Forward] |  |  |  |  |
Annual Effect on Future Equity Earnings in ATC |  |  |  | $ 0.9 |

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