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Equity Investments - Investment in ATC (Details) - USD ($) $ in Millions | 3 Months Ended | 6 Months Ended
--- | --- | ---
Jun. 30, 2023 | Jun. 30, 2022 | Jun. 30, 2023 | Jun. 30, 2022
ALLETE's Investment in ATC [Roll Forward] |  |  |  |
Equity Investment Balance as of December 31, 2022 |  |  | $ 322.7 |
Cash Investments |  |  | 4.3 | $ 3.9
Equity in ATC Earnings | $ 5.4 | $ 5.3 | 11.4 | $ 10.8
Equity Investment Balance as of June 30, 2023 | $ 327.5 |  | $ 327.5 |
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, 2022 |  |  | $ 165.4 |
Cash Investments |  |  | 4.3 |
Equity in ATC Earnings |  |  | 11.6 |
Distributed Earnings |  |  | (9.3) |
Amortization of the Remeasurement of Deferred Income Taxes |  |  | 0.7 |
Equity Investment Balance as of June 30, 2023 | $ 172.7 |  | $ 172.7 |
Authorized Return on Equity |  |  | 10.02% |
Authorized Return on Equity, Including Incentive Adder | 10.52% |  | 10.52% |
Approximate Annual 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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