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Investment in ATC (Details) - USD ($) $ in Millions |  | 3 Months Ended | 9 Months Ended
--- | --- | --- | ---
Oct. 31, 2018 | Sep. 30, 2018 | Sep. 30, 2017 | Sep. 30, 2018 | Sep. 30, 2017
ALLETE's Investment in ATC [Roll Forward] |  |  |  |  |  |
Equity Investment Balance as of December 31, 2017 |  |  |  |  | $ 118.7 |
Cash Investments |  |  |  |  | 5.0 | $ 6.6
Equity in ATC Earnings |  |  | $ 4.0 | $ 5.9 | 13.0 | $ 17.3
Equity Investment Balance as of September 30, 2018 |  |  | $ 126.0 |  | $ 126.0 |
FERC [Member] | Return on Equity Complaint [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 2018 |  |  | $ 0.0 |  | $ 0.0 |
ALLETE's Investment in ATC [Roll Forward] |  |  |  |  |  |
Equity Investment Balance as of December 31, 2017 |  |  |  |  | 118.7 |
Cash Investments |  |  |  |  | 5.0 |
Equity in ATC Earnings |  |  |  |  | 13.0 |
Distributed ATC Earnings |  |  |  |  | (11.7) |
Amortization of the Remeasurement of Deferred Income Taxes | [1] |  |  |  | 1.0 |
Equity Investment Balance as of September 30, 2018 |  |  | $ 126.0 |  | $ 126.0 |
Authorized Return on Equity |  |  | 10.32% |  | 10.32% |
Authorized Return on Equity, Including Incentive Adder |  |  | 10.82% |  | 10.82% |
ATC [Member] | Subsequent Event [Member] |  |  |  |  |  |
ALLETE's Investment in ATC [Roll Forward] |  |  |  |  |  |
Cash Investments |  | $ 1.2 |  |  |  |
[1] | Amortization related to the impact of the remeasurement of deferred income tax assets and liabilities resulting from the TCJA.

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