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Equity Investments - Investment in ATC (Details) - USD ($) $ in Millions | 3 Months Ended | 6 Months Ended
--- | --- | ---
Jun. 30, 2022 | Jun. 30, 2021 | Jun. 30, 2022 | Jun. 30, 2021
ALLETE's Investment in ATC [Roll Forward] |  |  |  |
Equity Investment Balance as of December 31, 2021 |  |  | $ 318.0 |
Cash Investments |  |  | 3.9 | $ 17.1
Equity in ATC Earnings | $ 5.3 | $ 5.1 | 10.8 | $ 9.9
Equity Investment Balance as of June 30, 2022 | $ 322.0 |  | $ 322.0 |
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, 2021 |  |  | $ 154.5 |
Cash Investments |  |  | 3.9 |
Equity in ATC Earnings |  |  | 10.7 |
Distributed Earnings |  |  | (8.6) |
Amortization of the Remeasurement of Deferred Income Taxes |  |  | 0.7 |
Equity Investment Balance as of June 30, 2022 | $ 161.2 |  | $ 161.2 |
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 |
Approximate Reduction in Pretax Income for FERC Proposal to Limit Incentive adder, ALLETE's Proportionate Share | $ 1.0 |  | $ 1.0 |
Basis Point Incentive adder | 50.00% |  | 50.00% |

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