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Short-Term and Long-Term Debt (Details) (USD $) In Millions, unless otherwise specified | 9 Months Ended |  |  | 0 Months Ended |  | 0 Months Ended |  | 0 Months Ended |  | 0 Months Ended |  | 0 Months Ended |
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Sep. 30, 2014 | Jan. 30, 2014 | Dec. 31, 2013 | Mar. 04, 2014 ALLETE Bonds 3.69% Due March 2024 [Member] | Mar. 04, 2014 ALLETE Bonds 4.95% Due March 2044 [Member] | Jun. 26, 2014 ALLETE Bonds 3.40% Due July 2022 [Member] | Jun. 26, 2014 ALLETE Bonds 3.40% Due July 2022 [Member] | Jun. 26, 2014 ALLETE Bonds 5.05% Due July 2044 [Member] | Jun. 26, 2014 ALLETE Bonds 5.05% Due July 2044 [Member] | Sep. 16, 2014 ALLETE Bonds 3.02% Due September 2021 [Member] | Sep. 16, 2014 ALLETE Bonds 3.02% Due September 2021 [Member] | Sep. 16, 2014 ALLETE Bonds 3.74% Due September 2029 [Member] | Sep. 16, 2014 ALLETE Bonds 3.74% Due September 2029 [Member] | Sep. 16, 2014 ALLETE Bonds 4.39% Due September 2044 [Member] | Sep. 16, 2014 ALLETE Bonds 4.39% Due September 2044 [Member]
Debt Instrument [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Short-Term Debt Outstanding | $ 88.0 |  | $ 27.2 |  |  |  |  |  |  |  |  |  |  |  |
Long-Term Debt Outstanding | 1,289.2 |  | 1,083.0 |  |  |  |  |  |  |  |  |  |  |  |
Business Combination, Recognized Identifiable Assets Acquired and Liabilities Assumed, Long-term Debt Including Long-term Debt Due Within One Year |  | 23.3 |  |  |  |  |  |  |  |  |  |  |  |  |
Business Combination, Recognized Identifiable Assets Acquired and Liabilities Assumed, Long-term Debt Due Within One Year |  | 2.2 |  |  |  |  |  |  |  |  |  |  |  |  |
Proceeds from Issuance of First Mortgage Bonds | $ 375.0 |  |  | $ 60.0 | $ 40.0 | $ 75.0 |  | $ 40.0 |  | $ 60.0 |  | $ 50.0 |  | $ 50.0 |
Interest Rate |  |  |  | 3.69% | 4.95% |  | 3.40% |  | 5.05% |  | 3.02% |  | 3.74% |  | 4.39%

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- Definition Amount of long-term debt due within one year or within the normal operating cycle, if longer, assumed at the acquisition date. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 805 -SubTopic 20 -Section 50 -Paragraph 1 -Subparagraph (c) -URI http://asc.fasb.org/extlink&oid=39739317&loc=d3e4845-128472 + Details
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- Definition Amount of short-term debt and current maturity of long-term debt and capital lease obligations due within one year or the normal operating cycle, if longer. + 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 210.5-02.19,20) -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 19 -Article 5 + Details
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- Definition Contractual interest rate for funds borrowed, under the debt agreement. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher SEC -Name Regulation S-X (SX) -Number 210 -Section 02 -Paragraph 22 -Article 5 Reference 2: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 210 -SubTopic 10 -Section S99 -Paragraph 1 -Subparagraph (SX 210.5-02.22(a)(1)) -URI http://asc.fasb.org/extlink&oid=6877327&loc=d3e13212-122682 + Details
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- Definition Carrying amount of long-term debt, net of unamortized discount or premium, excluding amounts to be repaid within one year or the normal operating cycle, if longer (current maturities). Includes, but not limited to, notes payable, bonds payable, debentures, mortgage loans and commercial paper. Excludes capital lease obligations. + 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 210.5-02.22) -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 22 -Article 5 + Details
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- Definition The cash inflow from amounts received from a long-term debt instrument secured by a first mortgage deed of trust, containing a pledge of real property. The lender has the highest claim on the property in case of default. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Glossary Financing Activities -URI http://asc.fasb.org/extlink&oid=6513228 Reference 2: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 230 -SubTopic 10 -Section 45 -Paragraph 14 -Subparagraph (b) -URI http://asc.fasb.org/extlink&oid=31042434&loc=d3e3255-108585 + Details
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