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Short-Term and Long-Term Debt - Long-Term Debt (Details) $ in Millions |  |  |  | 12 Months Ended |
--- | --- | --- | --- | --- | ---
Oct. 17, 2018 USD ($) | Oct. 03, 2018 USD ($) | Apr. 16, 2018 USD ($) | Dec. 31, 2018 USD ($) Years | Dec. 31, 2017 USD ($)
Debt Instrument [Line Items] |  |  |  |  |
Long-Term Debt |  |  |  | $ 1,428.5 | $ 1,439.2
Long-Term Debt - Unamortized Debt Issuance Costs |  |  |  | 8.8 |
Debt Instrument [Abstract] |  |  |  |  |
Long-Term Debt Maturing in 2019 |  |  |  | 57.9 |
Long-Term Debt Maturing in 2020 |  |  |  | 113.7 |
Long-Term Debt Maturing in 2021 |  |  |  | 98.5 |
Long-Term Debt Maturing in 2022 |  |  |  | 89.3 |
Long-Term Debt Maturing in 2023 |  |  |  | 88.5 |
Long-Term Debt Maturing Thereafter |  |  |  | 1,047.3 |
Camp Ripley Financing [Member] |  |  |  |  |
Debt Instrument [Abstract] |  |  |  |  |
Annual Financing Payment |  |  |  | $ 1.4 |
Financing Renewal Term (Years) | Years |  |  |  | 2 |
Purchase Option |  |  |  | $ 4.0 |
ALLETE Senior Unsecured Notes 3.11% Due 2027 [Member] |  |  |  |  |
Debt Instrument [Abstract] |  |  |  |  |
Interest Rate |  |  |  | 3.11% |
ALLETE Bonds 4.07% Due April 2048 [Member] |  |  |  |  |
Debt Instrument [Abstract] |  |  |  |  |
Proceeds from Issuance of First Mortgage Bond |  |  | $ 60.0 |  |
Interest Rate |  |  | 4.07% | 4.07% |
Unsecured Term Loan Due in 2020 [Member] |  |  |  |  |
Debt Instrument [Abstract] |  |  |  |  |
Repayments of Debt | $ 10.0 | $ 20.0 |  |  |

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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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