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Equity Investments - Investment in ATC (Details) - USD ($) $ in Millions | 3 Months Ended | 9 Months Ended
--- | --- | ---
Sep. 30, 2021 | Sep. 30, 2020 | Sep. 30, 2021 | Sep. 30, 2020
ALLETE's Investment in ATC [Roll Forward] |  |  |  |
Equity Investment Balance as of December 31, 2020 |  |  | $ 301.2 |
Cash Investments |  |  | 17.4 | $ 91.0
Equity in ATC Earnings | $ 4.4 | $ 5.1 | 14.3 | $ 16.7
Equity Investment Balance as of September 30, 2021 | $ 317.8 |  | $ 317.8 |
ATC [Member] |  |  |  |
Investment in ATC [Line Items] |  |  |  |
Ownership Percentage | 8.00% |  | 8.00% |
ALLETE's Investment in ATC [Roll Forward] |  |  |  |
Equity Investment Balance as of December 31, 2020 |  |  | $ 149.0 |
Equity in ATC Earnings |  |  | 15.8 |
Distributed Earnings |  |  | (12.8) |
Amortization of the Remeasurement of Deferred Income Taxes |  |  | 1.0 |
Equity Investment Balance as of September 30, 2021 | $ 153.0 |  | $ 153.0 |
Authorized Return on Equity |  |  | 10.02% |
Authorized Return on Equity, Including Incentive Adder | 10.52% |  | 10.52% |
Approximate Refund for Reversal of FERC ROE Order | $ 66.0 |  | $ 66.0 |
Approximate Refund for Reversal of FERC ROE Order, ALLETE's Proportion | $ 5.0 |  | $ 5.0 |

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