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Short-Term and Long-Term Debt (Details) - USD ($) $ in Millions | Aug. 25, 2017 | Jun. 01, 2017 | Sep. 30, 2017 | Dec. 31, 2016
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Debt Instrument [Line Items] |  |  |  |
Short-Term Debt - Principal |  |  | $ 64.6 | $ 188.3
Short-Term Debt - Unamortized Debt Issuance Costs |  |  | (0.5) | (0.6)
Short-Term Debt - Total |  |  | 64.1 | 187.7
Long-Term Debt - Principal |  |  | 1,454.4 | 1,380.8
Long-Term Debt - Unamortized Debt Issuance Costs |  |  | (9.8) | (10.4)
Long-Term Debt - Total |  |  | 1,444.6 | 1,370.4
Total Debt - Principal |  |  | 1,519.0 | 1,569.1
Total Debt - Unamortized Debt Issuance Costs |  |  | (10.3) | (11.0)
Total Debt - Total |  |  | $ 1,508.7 | $ 1,558.1
ALLETE Senior Unsecured Notes 3.11% Due 2027 [Member] |  |  |  |
Debt Instrument [Line Items] |  |  |  |
Proceeds from Issuance of Long-Term Debt |  | $ 80.0 |  |
Interest Rate |  | 3.11% |  |
ALLETE Term Loan Variable Rate Due 2020 [Member] |  |  |  |
Debt Instrument [Line Items] |  |  |  |
Proceeds from Issuance of Long-Term Debt | $ 40.0 |  |  |
Interest Rate - Basis | LIBOR |  |  |
Interest Rate - Spread | 1.025% |  |  |

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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 Percentage points added to the reference rate to compute the variable rate on the debt instrument. + References No definition available. + 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 The reference rate for the variable rate of the debt instrument, such as LIBOR or the US Treasury rate and the maturity of the reference rate used, such as three months or six months LIBOR. + References No definition available. + 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 Line items represent financial concepts included in a table. These concepts are used to disclose reportable information associated with domain members defined in one or many axes to the table. + 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. + References No definition available. + Details
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- Definition Amount after unamortized (discount) premium and debt issuance costs of long-term debt classified as noncurrent and excluding amounts to be repaid within one year or the normal operating cycle, if longer. 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 + Details
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