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Short-Term and Long-Term Debt (Details) - USD ($) $ in Millions | Mar. 01, 2019 | Mar. 31, 2019 | Jan. 10, 2019 | Dec. 31, 2018
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Debt Instrument [Line Items] |  |  |  |
Short-Term Debt - Principal |  | $ 14.7 |  | $ 57.9
Short-Term Debt - Unamortized Debt Issuance Costs |  | (0.4) |  | (0.4)
Short-Term Debt - Total |  | 14.3 |  | 57.5
Long-Term Debt - Principal |  | 1,534.3 |  | 1,437.3
Long-Term Debt - Unamortized Debt Issuance Costs |  | (9.3) |  | (8.8)
Long-Term Debt - Total |  | 1,525.0 |  | 1,428.5
Total Debt - Principal |  | 1,549.0 |  | 1,495.2
Total Debt - Unamortized Debt Issuance Costs |  | (9.7) |  | (9.2)
Total Debt - Total |  | $ 1,539.3 |  | $ 1,486.0
ALLETE Bonds 4.08% Due March 2029 [Member] |  |  |  |
Debt Instrument [Line Items] |  |  |  |
Proceeds from Issuance of First Mortgage Bond | $ 70.0 |  |  |
Interest Rate | 4.08% |  |  |
ALLETE Bonds 4.47% Due March 2049 [Member] |  |  |  |
Debt Instrument [Line Items] |  |  |  |
Proceeds from Issuance of First Mortgage Bond | $ 30.0 |  |  |
Interest Rate | 4.47% |  |  |
Revolving Credit Facility [Member] |  |  |  |
Debt Instrument [Line Items] |  |  |  |
Line of Credit Facility, Maximum Borrowing Capacity |  |  | $ 400.0 |
Line of Credit Facility Additional Borrowing Capacity |  |  | 150.0 |
Letters of Credit Maximum Issuances |  |  | $ 60.0 |

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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 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=109500613&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=6879938&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=6877327&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=6876686&loc=d3e534808-122878 + Details
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- Definition Contractual interest rate for funds borrowed, under the debt agreement. + 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(a)(1)) -URI http://asc.fasb.org/extlink&oid=6877327&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=109223946&loc=d3e3255-108585 + Details
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