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Short-Term and Long-Term Debt (Details) - USD ($) $ in Millions |  | 9 Months Ended |
--- | --- | --- | ---
Sep. 01, 2021 | Sep. 30, 2021 | Dec. 31, 2020
Debt Instrument [Line Items] |  |  |
Short-Term Debt - Principal |  | $ 377.6 | $ 204.0
Short-Term Debt - Unamortized Debt Issuance Costs |  | (0.2) | (0.3)
Short-Term Debt - Total |  | 377.4 | 203.7
Long-Term Debt - Principal |  | 1,658.5 | 1,602.4
Long-Term Debt - Unamortized Debt Issuance Costs |  | (9.1) | (9.2)
Long-Term Debt - Total |  | 1,649.4 | 1,593.2
Total Debt - Principal |  | 2,036.1 | 1,806.4
Total Debt - Unamortized Debt Issuance Costs |  | (9.3) | (9.5)
Total Debt - Total |  | 2,026.8 | 1,796.9
Unsecured Term Loan Due in 2022 |  |  |
Debt Instrument [Line Items] |  |  |
Unsecured Term Loan, Amount |  | $ 150.0 |
Variable Rate Basis |  | LIBOR |
Basis Spread on Variable Rate |  | 0.75% |
Debt Drawn Through Accordion Feature |  | $ 35.0 |
Unsecured Debt, Total Drawn |  | 185.0 |
ALLETE Bonds 2.79% Due September 2031 |  |  |
Debt Instrument [Line Items] |  |  |
Proceeds from Issuance of First Mortgage Bond | $ 100.0 |  |
Interest Rate | 2.79% |  |
Line of Credit [Member] |  |  |
Debt Instrument [Line Items] |  |  |
Letters of Credit Outstanding, Amount |  | 26.2 | 22.3
Line of Credit, Current |  | $ 25.2 | $ 0.0

X
- 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 Debt Drawn Through Accordion Feature + 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 Unsecured Debt, Total Drawn on Loan + References No definition available. + Details
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- Definition Unsecured Term Loan, Amount + 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://fasb.org/us-gaap/role/ref/legacyRef -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=120391452&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/disclosureRef -Publisher FASB -Name Accounting Standards Codification -Topic 470 -SubTopic 20 -Section 50 -Paragraph 4 -Subparagraph (b)(1) -URI http://asc.fasb.org/extlink&oid=123466204&loc=SL6031897-161870 Reference 2: http://fasb.org/us-gaap/role/ref/legacyRef -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=120400017&loc=d3e572229-122910 Reference 3: http://fasb.org/us-gaap/role/ref/legacyRef -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=120391452&loc=d3e13212-122682 Reference 4: http://fasb.org/us-gaap/role/ref/legacyRef -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=120398452&loc=d3e534808-122878 + Details
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- Definition Description of reference rate used for variable rate of debt instrument. + 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/disclosureRef -Publisher FASB -Name Accounting Standards Codification -Topic 470 -SubTopic 20 -Section 50 -Paragraph 1B -Subparagraph (b) -URI http://asc.fasb.org/extlink&oid=123466505&loc=SL123495323-112611 Reference 2: http://fasb.org/us-gaap/role/ref/legacyRef -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=120391452&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 The total amount of the contingent obligation under letters of credit outstanding as of the reporting date. + References No definition available. + Details
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- Definition The carrying value as of the balance sheet date of the current portion of long-term obligations drawn from a line of credit, which is a bank's commitment to make loans up to a specific amount. Examples of items that might be included in the application of this element may consist of letters of credit, standby letters of credit, and revolving credit arrangements, under which borrowings can be made up to a maximum amount as of any point in time conditional on satisfaction of specified terms before, as of and after the date of drawdowns on the line. Includes short-term obligations that would normally be classified as current liabilities but for which (a) postbalance sheet date issuance of a long term obligation to refinance the short term obligation on a long term basis, or (b) the enterprise has entered into a financing agreement that clearly permits the enterprise to refinance the short-term obligation on a long term basis and the following conditions are met (1) the agreement does not expire within 1 year and is not cancelable by the lender except for violation of an objectively determinable provision, (2) no violation exists at the BS date, and (3) the lender has entered into the financing agreement is expected to be financially capable of honoring the agreement. + References Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef -Publisher FASB -Name Accounting Standards Codification -Topic 942 -SubTopic 210 -Section S99 -Paragraph 1 -Subparagraph (SX 210.9-03(13)) -URI http://asc.fasb.org/extlink&oid=120398452&loc=d3e534808-122878 Reference 2: http://fasb.org/us-gaap/role/ref/legacyRef -Publisher FASB -Name Accounting Standards Codification -Topic 210 -SubTopic 10 -Section S99 -Paragraph 1 -Subparagraph (SX 210.5-02(19)) -URI http://asc.fasb.org/extlink&oid=120391452&loc=d3e13212-122682 + 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://fasb.org/us-gaap/role/ref/legacyRef -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=120391452&loc=d3e13212-122682 + 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://fasb.org/us-gaap/role/ref/legacyRef -Publisher FASB -Name Accounting Standards Codification -Topic 230 -SubTopic 10 -Section 45 -Paragraph 14 -Subparagraph (b) -URI http://asc.fasb.org/extlink&oid=123570139&loc=d3e3255-108585 + 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 the respective bond issues. This does not include the amounts capitalized as part of the cost of the utility plant or asset. + References Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef -Publisher FASB -Name Accounting Standards Codification -Topic 210 -SubTopic 10 -Section S99 -Paragraph 1 -Subparagraph (SX 210.5-02.17) -URI http://asc.fasb.org/extlink&oid=120391452&loc=d3e13212-122682 + Details
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