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Earnings Per Share and Common Stock (Tables) | 9 Months Ended
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Sep. 30, 2017
Earnings Per Share [Abstract] |
Reconciliation of Basic and Diluted Earnings Per Share [Table Text Block] |
|  |  | 2017 |  |  |  |  |  | 2016 |  |
Reconciliation of Basic and Diluted |  |  | Dilutive |  |  |  |  |  | Dilutive |  |
Earnings Per Share | Basic |  | Securities |  | Diluted |  | Basic |  | Securities |  | Diluted
Millions Except Per Share Amounts |  |  |  |  |  |  |  |  |  |  |
Quarter ended September 30, |  |  |  |  |  |  |  |  |  |  |
Net Income Attributable to ALLETE |  | $44.9 |  |  |  |  |  | $44.9 |  |  |  | $40.3 |  |  |  |  |  | $40.3 |
Average Common Shares | 51.0 |  |  | 0.2 |  |  | 51.2 |  |  | 49.4 |  |  | 0.1 |  |  | 49.5 |
Earnings Per Share |  | $0.88 |  |  |  |  |  | $0.88 |  |  |  | $0.82 |  |  |  |  |  | $0.81 |
Nine Months Ended September 30, |  |  |  |  |  |  |  |  |  |  |  |  |
Net Income Attributable to ALLETE |  | $130.8 |  |  |  |  |  | $130.8 |  |  |  | $111.0 |  |  |  |  |  | $111.0 |
Average Common Shares | 50.7 |  |  | 0.2 |  |  | 50.9 |  |  | 49.3 |  |  | 0.1 |  |  | 49.4 |
Earnings Per Share |  | $2.58 |  |  |  |  |  | $2.57 |  |  |  | $2.25 |  |  |  |  |  | $2.25 |

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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/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 260 -SubTopic 10 -Section 50 -Paragraph 1 -URI http://asc.fasb.org/extlink&oid=6371337&loc=d3e3550-109257 + Details
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