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Investment in ATC (Details) $ in Millions |  | 3 Months Ended | 6 Months Ended |
--- | --- | --- | --- | ---
Jul. 31, 2017 USD ($) | Jun. 30, 2017 USD ($) | Jun. 30, 2016 USD ($) | Jun. 30, 2017 USD ($) | Jun. 30, 2016 USD ($) | Sep. 30, 2016
ALLETE's Investment in ATC [Roll Forward] |  |  |  |  |  |
Equity Investment Balance as of December 31, 2016 |  |  |  | $ 135.6 |  |
Cash Investments |  |  |  | 5.0 | $ 1.6 |
Equity in ATC Earnings |  | $ 5.3 | $ 4.1 | 11.4 | $ 8.9 |
Equity Investment Balance as of June 30, 2017 |  | $ 143.1 |  | $ 143.1 |  |
FERC [Member] | Return on Equity Complaint 2 [Member] |  |  |  |  |  |
ALLETE's Investment in ATC [Roll Forward] |  |  |  |  |  |
Proposed Return on Equity |  | 9.70% |  | 9.70% |  |
Proposed Return on Equity, Including Incentive Adder |  | 10.20% |  | 10.20% |  |
ATC [Member] |  |  |  |  |  |
Investment in ATC [Line Items] |  |  |  |  |  |
Ownership Percentage |  | 8.00% |  | 8.00% |  |
Expected Additional Investment in 2017 |  | $ 4.3 |  | $ 4.3 |  |
ALLETE's Investment in ATC [Roll Forward] |  |  |  |  |  |
Equity Investment Balance as of December 31, 2016 |  |  |  | 135.6 |  |
Cash Investments |  |  |  | 5.0 |  |
Equity in ATC Earnings |  |  |  | 11.4 |  |
Distributed ATC Earnings |  |  |  | (8.9) |  |
Equity Investment Balance as of June 30, 2017 |  | $ 143.1 |  | $ 143.1 |  |
Authorized Return on Equity |  | 10.32% |  | 10.32% |  | 12.20%
Authorized Return on Equity, Including Incentive Adder |  | 10.82% |  | 10.82% |  |
ATC [Member] | Sensitivity Analysis [Member] |  |  |  |  |  |
ALLETE's Investment in ATC [Roll Forward] |  |  |  |  |  |
Basis Point Reduction on Approved Rate of Return on Common Equity |  | 50 |  | 50 |  |
ATC [Member] | Sensitivity Analysis [Member] | After-tax [Member] |  |  |  |  |  |
ALLETE's Investment in ATC [Roll Forward] |  |  |  |  |  |
Annual Effect on Future Equity Earnings in ATC |  |  |  | $ 0.5 |  |
ATC [Member] | Subsequent Event [Member] |  |  |  |  |  |
ALLETE's Investment in ATC [Roll Forward] |  |  |  |  |  |
Cash Investments | $ 1.6 |  |  |  |  |

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