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Earnings Per Share and Common Stock (Tables) | 3 Months Ended
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Mar. 31, 2021
Earnings Per Share [Abstract] |
Reconciliation of Basic and Diluted Earnings Per Share [Table Text Block] |
|  |  | 2021 |  |  |  |  |  | 2020 |  |
Reconciliation of Basic and Diluted |  |  | Dilutive |  |  |  |  |  | Dilutive |  |
Earnings Per Share | Basic |  | Securities |  | Diluted |  | Basic |  | Securities |  | Diluted
Millions Except Per Share Amounts |  |  |  |  |  |  |  |  |  |  |
Three Months Ended March 31, |  |  |  |  |  |  |  |  |  |  |
Net Income Attributable to ALLETE | $51.8 |  |  |  |  | $51.8 |  |  | $66.3 |  |  |  |  | $66.3 |
Average Common Shares | 52.1 |  |  | 0.1 |  |  | 52.2 |  |  | 51.7 |  |  | 0.1 |  |  | 51.8 |
Earnings Per Share | $0.99 |  |  |  |  | $0.99 |  |  | $1.28 |  |  |  |  | $1.28 |

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- Definition Tabular disclosure of an entity's basic and diluted earnings per share calculations, including a reconciliation of numerators and denominators of the basic and diluted per-share computations for income from continuing operations. + References Reference 1: http://www.xbrl.org/2009/role/commonPracticeRef -Publisher FASB -Name Accounting Standards Codification -Topic 260 -SubTopic 10 -Section 50 -Paragraph 1 -Subparagraph (a) -URI http://asc.fasb.org/extlink&oid=6371337&loc=d3e3550-109257 + Details
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