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Investment in ATC (Details) - USD ($) $ in Millions |  | 3 Months Ended
--- | --- | ---
Apr. 30, 2019 | Mar. 31, 2019 | Mar. 31, 2018
ALLETE's Investment in ATC [Roll Forward] |  |  |
Equity Investment Balance as of December 31, 2018 |  | $ 161.1 |
Cash Investments |  | 0.5 | $ 1.6
Equity in ATC Earnings |  | 5.6 | $ 4.7
Equity Investment Balance as of March 31, 2019 |  | $ 154.8 |
FERC [Member] | Return on Equity Complaint [Member] |  |  |
ALLETE's Investment in ATC [Roll Forward] |  |  |
Proposed Return on Equity |  | 9.70% |
Proposed Return on Equity, Including Incentive Adder |  | 10.20% |
ATC [Member] |  |  |
Investment in ATC [Line Items] |  |  |
Ownership Percentage |  | 8.00% |
Expected Additional Investment in 2019 |  | $ 5.8 |
ALLETE's Investment in ATC [Roll Forward] |  |  |
Equity Investment Balance as of December 31, 2018 |  | 128.1 |
Cash Investments |  | 0.4 |
Equity in ATC Earnings |  | 5.6 |
Distributed ATC Earnings |  | (4.4) |
Amortization of the Remeasurement of Deferred Income Taxes |  | 0.3 |
Equity Investment Balance as of March 31, 2019 |  | $ 130.0 |
Authorized Return on Equity |  | 10.32% |
Authorized Return on Equity, Including Incentive Adder |  | 10.82% |
ATC [Member] | Subsequent Event [Member] |  |  |
ALLETE's Investment in ATC [Roll Forward] |  |  |
Cash Investments | $ 2.3 |  |

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