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Short-Term and Long-Term Debt - Schedule of Long-Term Debt (Details) - USD ($) $ in Millions | Dec. 31, 2021 | Dec. 31, 2020
--- | --- | ---
Debt Instrument [Line Items] |  |  |
Long-term Debt | [1] | $ 1,986.4 | $ 1,806.4
Unamortized Debt Issuance Costs |  | (9.0) | (9.5)
Total Long-Term Debt |  | 1,977.4 | 1,796.9
Less: Due Within One Year |  | 214.2 | 203.7
Net Long-Term Debt |  | $ 1,763.2 | 1,593.2
First Mortgage Bonds - 4.85% Series Due 2021 [Member] |  |  |
Debt Instrument [Line Items] |  |  |
Interest Rate |  | 4.85% |
Long-term Debt |  | $ 0.0 | 15.0
First Mortgage Bonds - 3.02% Series Due 2021 [Member] |  |  |
Debt Instrument [Line Items] |  |  |
Interest Rate |  | 3.02% |
Long-term Debt |  | $ 0.0 | 60.0
First Mortgage Bonds - 3.40% Series Due 2022 [Member] |  |  |
Debt Instrument [Line Items] |  |  |
Interest Rate |  | 3.40% |
Long-term Debt |  | $ 75.0 | 75.0
First Mortgage Bonds - 6.02% Series Due 2023 [Member] |  |  |
Debt Instrument [Line Items] |  |  |
Interest Rate |  | 6.02% |
Long-term Debt |  | $ 75.0 | 75.0
First Mortgage Bonds - 3.69% Series Due 2024 [Member] |  |  |
Debt Instrument [Line Items] |  |  |
Interest Rate |  | 3.69% |
Long-term Debt |  | $ 60.0 | 60.0
First Mortgage Bonds - 4.90% Series Due 2025 [Member] |  |  |
Debt Instrument [Line Items] |  |  |
Interest Rate |  | 4.90% |
Long-term Debt |  | $ 30.0 | 30.0
First Mortgage Bonds - 5.10% Series Due 2025 [Member] |  |  |
Debt Instrument [Line Items] |  |  |
Interest Rate |  | 5.10% |
Long-term Debt |  | $ 30.0 | 30.0
First Mortgage Bonds - 3.20% Series Due 2026 [Member] |  |  |
Debt Instrument [Line Items] |  |  |
Interest Rate |  | 3.20% |
Long-term Debt |  | $ 75.0 | 75.0
First Mortgage Bonds - 5.99% Series Due 2027 [Member] |  |  |
Debt Instrument [Line Items] |  |  |
Interest Rate |  | 5.99% |
Long-term Debt |  | $ 60.0 | 60.0
First Mortgage Bonds - 3.30% Series Due 2028 [Member] |  |  |
Debt Instrument [Line Items] |  |  |
Interest Rate |  | 3.30% |
Long-term Debt |  | $ 40.0 | 40.0
First Mortgage Bonds - 2.50% Due August 2030 [Member] |  |  |
Debt Instrument [Line Items] |  |  |
Interest Rate |  | 4.08% |
Long-term Debt |  | $ 70.0 | 70.0
First Mortgage Bonds - 3.74% Series Due 2029 [Member] |  |  |
Debt Instrument [Line Items] |  |  |
Interest Rate |  | 3.74% |
Long-term Debt |  | $ 50.0 | 50.0
First Mortgage Bonds - 2.50% Series Due 2030 |  |  |
Debt Instrument [Line Items] |  |  |
Interest Rate |  | 2.50% |
Long-term Debt |  | $ 46.0 | 46.0
First Mortgage Bonds - 3.86% Series Due 2030 [Member] |  |  |
Debt Instrument [Line Items] |  |  |
Interest Rate |  | 3.86% |
Long-term Debt |  | $ 60.0 | 60.0
First Mortgage Bonds - 2.79% Series Due 2031 |  |  |
Debt Instrument [Line Items] |  |  |
Interest Rate |  | 2.79% |
Long-term Debt |  | $ 100.0 | 0.0
First Mortgage Bonds - 5.69% Series Due 2036 [Member] |  |  |
Debt Instrument [Line Items] |  |  |
Interest Rate |  | 5.69% |
Long-term Debt |  | $ 50.0 | 50.0
First Mortgage Bonds - 6.00% Series Due 2040 [Member] |  |  |
Debt Instrument [Line Items] |  |  |
Interest Rate |  | 6.00% |
Long-term Debt |  | $ 35.0 | 35.0
First Mortgage Bonds - 5.82% Series Due 2040 [Member] |  |  |
Debt Instrument [Line Items] |  |  |
Interest Rate |  | 5.82% |
Long-term Debt |  | $ 45.0 | 45.0
First Mortgage Bonds - 4.08% Series Due 2042 [Member] |  |  |
Debt Instrument [Line Items] |  |  |
Interest Rate |  | 4.08% |
Long-term Debt |  | $ 85.0 | 85.0
First Mortgage Bonds - 4.21% Series Due 2043 [Member] |  |  |
Debt Instrument [Line Items] |  |  |
Interest Rate |  | 4.21% |
Long-term Debt |  | $ 60.0 | 60.0
First Mortgage Bonds - 4.95% Series Due 2044 [Member] |  |  |
Debt Instrument [Line Items] |  |  |
Interest Rate |  | 4.95% |
Long-term Debt |  | $ 40.0 | 40.0
First Mortgage Bonds - 5.05% Series Due 2044 [Member] |  |  |
Debt Instrument [Line Items] |  |  |
Interest Rate |  | 5.05% |
Long-term Debt |  | $ 40.0 | 40.0
First Mortgage Bonds - 4.39% Series Due 2044 [Member] |  |  |
Debt Instrument [Line Items] |  |  |
Interest Rate |  | 4.39% |
Long-term Debt |  | $ 50.0 | 50.0
First Mortgage Bonds - 4.07% Series Due 2048 [Member] |  |  |
Debt Instrument [Line Items] |  |  |
Interest Rate |  | 4.07% |
Long-term Debt |  | $ 60.0 | 60.0
First Mortgage Bonds - 4.47% Series Due 2049 [Member] |  |  |
Debt Instrument [Line Items] |  |  |
Interest Rate |  | 4.47% |
Long-term Debt |  | $ 30.0 | 30.0
First Mortgage Bonds - 3.30% Series Due 2050 |  |  |
Debt Instrument [Line Items] |  |  |
Interest Rate |  | 3.30% |
Long-term Debt |  | $ 94.0 | 94.0
ALLETE Clean Energy Unsecured Term Loan Variable Rate Due 2021 |  |  |
Debt Instrument [Line Items] |  |  |
Long-term Debt |  | $ 0.0 | 65.0
Armenia Mountain Senior Secured Notes 3.26% Due 2024 [Member] |  |  |
Debt Instrument [Line Items] |  |  |
Interest Rate |  | 3.26% |
Long-term Debt |  | $ 29.1 | 38.6
Industrial Development Variable Rate Demand Refunding Revenue Bonds Series 2006, Due 2025 [Member] |  |  |
Debt Instrument [Line Items] |  |  |
Long-term Debt |  | 27.8 | 27.8
Revolving Credit Facility Variable Rate Due 2025 |  |  |
Debt Instrument [Line Items] |  |  |
Long-term Debt |  | $ 145.0 | 0.0
Unsecured Senior Notes 2.65% Due 2025 |  |  |
Debt Instrument [Line Items] |  |  |
Interest Rate |  | 2.65% |
Long-term Debt |  | $ 150.0 | 150.0
ALLETE Senior Unsecured Notes 3.11% Due 2027 [Member] |  |  |
Debt Instrument [Line Items] |  |  |
Interest Rate |  | 3.11% |
Long-term Debt |  | $ 80.0 | 80.0
SWL&P First Mortgage Bonds 4.15% Series Due 2028 [Member] |  |  |
Debt Instrument [Line Items] |  |  |
Interest Rate |  | 4.15% |
Long-term Debt |  | $ 15.0 | 15.0
SWL&P First Mortgage Bonds 4.14% Series Due 2048 [Member] |  |  |
Debt Instrument [Line Items] |  |  |
Interest Rate |  | 4.14% |
Long-term Debt |  | $ 12.0 | 12.0
Unsecured Term Loan Variable Rate Due 2021 |  |  |
Debt Instrument [Line Items] |  |  |
Long-term Debt |  | 0.0 | 40.0
Unsecured Term Loan Variable Rate Due 2022 |  |  |
Debt Instrument [Line Items] |  |  |
Long-term Debt |  | 110.0 | 0.0
Other Long-Term Debt, 2.16% – 4.58% Due 2022 – 2051 [Member] |  |  |
Debt Instrument [Line Items] |  |  |
Long-term Debt |  | $ 57.5 | $ 43.0
Other Long-Term Debt, 2.16% – 4.58% Due 2022 – 2051 [Member] | Minimum [Member] |  |  |
Debt Instrument [Line Items] |  |  |
Interest Rate |  | 2.16% |
Other Long-Term Debt, 2.16% – 4.58% Due 2022 – 2051 [Member] | Maximum [Member] |  |  |
Debt Instrument [Line Items] |  |  |
Interest Rate |  | 4.58% |
[1] | Excludes unamortized debt issuance costs.

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