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Short-Term and Long-Term Debt (Details) - USD ($) $ in Millions | Jun. 01, 2017 | Jun. 30, 2017 | Dec. 31, 2016
--- | --- | --- | ---
Debt Instrument [Line Items] |  |  |
Short-Term Debt - Principal |  | $ 118.2 | $ 188.3
Short-Term Debt - Unamortized Debt Issuance Costs |  | (0.5) | (0.6)
Short-Term Debt - Total |  | 117.7 | 187.7
Long-term Debt - Principal |  | 1,411.4 | 1,380.8
Long-Term Debt - Unamortized Debt Issuance Costs |  | (10.0) | (10.4)
Long-Term Debt - Total |  | 1,401.4 | 1,370.4
Total Debt - Principal |  | 1,529.6 | 1,569.1
Total Debt - Unamortized Debt Issuance Costs |  | (10.5) | (11.0)
Total Debt - Total |  | $ 1,519.1 | $ 1,558.1
ALLETE Senior Unsecured Notes 3.11% Due 2027 [Member] |  |  |
Debt Instrument [Line Items] |  |  |
Proceeds from Issuance of Senior Unsecured Notes | $ 80.0 |  |
Interest Rate |  | 3.11% |

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- Definition Amount of short-term debt and current maturities of long-term debt before deduction of unamortized discount or premium and unamortized debt issuance costs. + References No definition available. + Details
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- Definition Represents the aggregate of total long-term debt, including current maturities and short-term debt, before deduction of unamortized discount or premium and unamortized debt issuance expense. + References No definition available. + Details
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- Definition The remaining balance of debt issuance expenses that were capitalized and are being amortized against income over the lives of short-term debt or the current maturities of long-term debt. + References No definition available. + Details
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- Definition The remaining balance of debt issuance expenses that were capitalized and are being amortized against income over the lives of long-term debt excluding current maturities. + References No definition available. + 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 + Details
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- Definition Amount, before unamortized (discount) premium and debt issuance costs, of long-term debt. Includes, but is not limited to, notes payable, bonds payable, commercial loans, mortgage loans, convertible debt, subordinated debt and other types of debt. + 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 FASB -Name Accounting Standards Codification -Topic 944 -SubTopic 210 -Section S99 -Paragraph 1 -Subparagraph (SX 210.7-03(16)) -URI http://asc.fasb.org/extlink&oid=6879938&loc=d3e572229-122910 Reference 3: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 942 -SubTopic 210 -Section S99 -Paragraph 1 -Subparagraph (SX 210.9-03(16)) -URI http://asc.fasb.org/extlink&oid=6876686&loc=d3e534808-122878 + 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 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 Represents the aggregate of total long-term debt, including current maturities and short-term debt. + References No definition available. + Details
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