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Short-Term and Long-Term Debt (Details) - USD ($) $ in Millions | Jul. 14, 2015 | Jul. 01, 2015 | Jun. 30, 2015 | Dec. 31, 2014
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Short-Term and Long-Term Debt [Line Items] |  |  |  |
Short-Term Debt Outstanding |  |  | $ 118.0 | $ 104.4
Long-Term Debt Outstanding |  |  | $ 1,272.4 | $ 1,272.8
Subsequent Event [Member] |  |  |  |
Short-Term and Long-Term Debt [Line Items] |  |  |  |
Expected Proceeds from the Issuance of First Mortgage Bonds | $ 100.0 |  |  |
Subsequent Event [Member] | ALLETE Bonds 2.80% Due September 2020 [Member] |  |  |  |
Short-Term and Long-Term Debt [Line Items] |  |  |  |
Expected Proceeds from the Issuance of First Mortgage Bonds | $ 40.0 |  |  |
Expected Interest Rate | 2.80% |  |  |
Subsequent Event [Member] | ALLETE Bonds 3.86% Due September 2030 [Member] |  |  |  |
Short-Term and Long-Term Debt [Line Items] |  |  |  |
Expected Proceeds from the Issuance of First Mortgage Bonds | $ 60.0 |  |  |
Expected Interest Rate | 3.86% |  |  |
Subsequent Event [Member] | Armenia Mountain [Member] |  |  |  |
Short-Term and Long-Term Debt [Line Items] |  |  |  |
Business Combination, Long-Term Debt Assumed Including Current Portion |  | $ 60.9 |  |
Business Combination, Current Portion of Long-Term Debt Assumed |  | $ 5.9 |  |

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- Definition Business Combination, Recognized Identifiable Assets Acquired and Liabilities Assumed, Long-term Debt, Including current portion + References No definition available. + Details
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- Definition The expected cash inflow from amounts received from a future long-term debt instrument issuance 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 No definition available. + Details
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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 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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