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Equity Investments - Investment in ATC (Details) - USD ($) $ in Millions | 3 Months Ended
--- | ---
Mar. 31, 2022 | Mar. 31, 2021
ALLETE's Investment in ATC [Roll Forward] |  |
Equity Investment Balance as of December 31, 2021 | $ 318.0 |
Cash Investments | 2.7 | $ 1.0
Equity in ATC Earnings | 5.5 | $ 4.8
Equity Investment Balance as of March 31, 2022 | $ 320.6 |
ATC [Member] |  |
Investment in ATC [Line Items] |  |
Ownership Percentage | 8.00% |
ALLETE's Investment in ATC [Roll Forward] |  |
Equity Investment Balance as of December 31, 2021 | $ 154.5 |
Cash Investments | 2.7 |
Equity in ATC Earnings | 5.4 |
Distributed Earnings | (4.4) |
Amortization of the Remeasurement of Deferred Income Taxes | 0.3 |
Equity Investment Balance as of March 31, 2022 | $ 158.5 |
Authorized Return on Equity | 10.02% |
Authorized Return on Equity, Including Incentive Adder | 10.52% |
Approximate Refund for Reversal of FERC ROE Order | $ 66.0 |
Approximate Refund for Reversal of FERC ROE Order, ALLETE's Proportion | 5.0 |
Approximate Reduction in Pretax Income for FERC Proposal to Limit Incentive adder, ALLETE's Proportionate Share | $ 1.0 |
Basis Point Incentive adder | 50.00% |

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