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Short-Term and Long-Term Debt (Details) - USD ($) $ in Millions | Oct. 17, 2018 | Oct. 03, 2018 | Apr. 16, 2018 | Sep. 30, 2018 | Dec. 31, 2017
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Debt Instrument [Line Items] |  |  |  |  |
Short-Term Debt - Principal |  |  |  | $ 57.0 | $ 64.6
Short-Term Debt - Unamortized Debt Issuance Costs |  |  |  | (0.4) | (0.5)
Short-Term Debt - Total |  |  |  | 56.6 | 64.1
Long-Term Debt - Principal |  |  |  | 1,470.9 | 1,448.7
Long-Term Debt - Unamortized Debt Issuance Costs |  |  |  | (9.2) | (9.5)
Long-Term Debt - Total |  |  |  | 1,461.7 | 1,439.2
Total Debt - Principal |  |  |  | 1,527.9 | 1,513.3
Total Debt - Unamortized Debt Issuance Costs |  |  |  | (9.6) | (10.0)
Total Debt - Total |  |  |  | $ 1,518.3 | $ 1,503.3
ALLETE First Mortgage Bonds 4.07% Due April 2048 [Member] |  |  |  |  |
Debt Instrument [Line Items] |  |  |  |  |
First Mortgage Bonds - Proceeds from Sale |  |  | $ 60.0 |  |
First Mortgage Bonds - Interest Rate |  |  | 4.07% |  |
Unsecured Term Loan Due in 2020 [Member] | Subsequent Event [Member] |  |  |  |  |
Debt Instrument [Line Items] |  |  |  |  |
Unsecured Term Loan Payment | $ 10.0 | $ 20.0 |  |  |

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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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