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Earnings Per Share and Common Stock (Details) - USD ($) $ / shares in Units, shares in Millions, $ in Millions |  |  |  |  | 3 Months Ended | 9 Months Ended
--- | --- | --- | --- | --- | --- | ---
Feb. 04, 2015 | Sep. 05, 2014 | Mar. 04, 2014 | Feb. 26, 2014 | Sep. 30, 2015 | Sep. 30, 2014 | Sep. 30, 2015 | Sep. 30, 2014
Earnings Per Share and Common Stock [Abstract] |  |  |  |  |  |  |  |
Antidilutive Shares Excluded from Diluted EPS Computation |  |  |  |  |  |  | 0.0 | 0.0
Earnings Per Share - Basic [Abstract] |  |  |  |  |  |  |  |
Net Income Attributable to ALLETE |  |  |  |  | $ 60.4 | $ 41.6 | $ 122.8 | $ 91.9
Average Common Shares |  |  |  |  | 48.8 | 42.9 | 48.0 | 42.1
Earnings Per Share |  |  |  |  | $ 1.24 | $ 0.97 | $ 2.56 | $ 2.18
Earnings Per Share - Diluted [Abstract] |  |  |  |  |  |  |  |
Net Income to Attributable ALLETE |  |  |  |  | $ 60.4 | $ 41.6 | $ 122.8 | $ 91.9
Average Common Shares |  |  |  |  | 48.9 | 42.9 | 48.1 | 42.3
Earnings Per Share |  |  |  |  | $ 1.23 | $ 0.97 | $ 2.55 | $ 2.17
Dilutive Securities (in shares) |  |  |  |  | 0.1 | 0.0 | 0.1 | 0.2
Proceeds from Issuance of Common Stock - Pension Plan |  |  |  |  |  |  | $ 0.0 | $ 19.5
Common Stock Issued - Pension Plan (in shares) |  |  |  |  |  |  |  | 0.4
Forward Contracts [Member] |  |  |  |  |  |  |  |
Forward Contract Indexed to Issuer's Equity [Line Items] |  |  |  |  |  |  |  |
Forward Contract Indexed to Issuer's Equity, Indexed Shares |  |  |  | 2.8 |  |  |  |
Forward Contract Indexed to Issuer's Equity, Forward Rate Per Share |  |  |  | $ 48.01 |  |  |  |
Common Stock Issued - Forward Contract (in shares) | 1.4 | 1.4 |  |  |  |  |  |
Proceeds from Issuance of Common Stock - Forward Contract | $ 65.4 | $ 65.0 |  |  |  |  |  |
Options Held [Member] |  |  |  |  |  |  |  |
Forward Contract Indexed to Issuer's Equity [Line Items] |  |  |  |  |  |  |  |
Common Stock Issued - Forward Contract (in shares) |  |  | 0.4 |  |  |  |  |
Proceeds from Issuance of Common Stock - Forward Contract |  |  | $ 20.2 |  |  |  |  |

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- Definition Securities (including those issuable pursuant to contingent stock agreements) that could potentially dilute basic earnings per share (EPS) or earnings per unit (EPU) in the future that were not included in the computation of diluted EPS or EPU because to do so would increase EPS or EPU amounts or decrease loss per share or unit amounts for the period presented. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Glossary Antidilution -URI http://asc.fasb.org/extlink&oid=6505113 Reference 2: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Glossary Diluted Earnings Per Share -URI http://asc.fasb.org/extlink&oid=6510752 Reference 3: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Glossary Contingent Stock Agreement -URI http://asc.fasb.org/extlink&oid=6508534 Reference 4: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 260 -SubTopic 10 -Section 50 -Paragraph 1 -Subparagraph (c) -URI http://asc.fasb.org/extlink&oid=6371337&loc=d3e3550-109257 + Details
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- Definition Total number of common shares of an entity that have been sold or granted to shareholders (includes common shares that were issued, repurchased and remain in the treasury). These shares represent capital invested by the firm's shareholders and owners, and may be all or only a portion of the number of shares authorized. Shares issued include shares outstanding and shares held in the treasury. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 210 -SubTopic 10 -Section S99 -Paragraph 1 -Subparagraph (SX 210.5-02.29) -URI http://asc.fasb.org/extlink&oid=6877327&loc=d3e13212-122682 Reference 2: http://www.xbrl.org/2003/role/presentationRef -Publisher SEC -Name Regulation S-X (SX) -Number 210 -Section 02 -Paragraph 30 -Article 5 + Details
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- Definition The amount of net income (loss) for the period available to each share of common stock or common unit outstanding during the reporting period and to each share or unit that would have been outstanding assuming the issuance of common shares or units for all dilutive potential common shares or units outstanding during the reporting period. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 225 -SubTopic 10 -Section S99 -Paragraph 2 -Subparagraph (SX 210.5-03.21) -URI http://asc.fasb.org/extlink&oid=26872669&loc=d3e20235-122688 Reference 2: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 260 -SubTopic 10 -Section 45 -Paragraph 2 -URI http://asc.fasb.org/extlink&oid=7655603&loc=d3e1252-109256 Reference 3: http://www.xbrl.org/2003/role/presentationRef -Publisher SEC -Name Regulation S-X (SX) -Number 210 -Section 04 -Paragraph 18 -Article 7 Reference 4: http://www.xbrl.org/2003/role/presentationRef -Publisher SEC -Name Regulation S-X (SX) -Number 210 -Section 03 -Paragraph 20 -Article 5 Reference 5: http://www.xbrl.org/2003/role/presentationRef -Publisher SEC -Name Regulation S-X (SX) -Number 210 -Section 04 -Paragraph 21 -Article 9 + Details
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