Short-Term and Long-Term Debt - Schedule of Long-Term Debt (Details) - USD ($) $ in Millions		12 Months Ended
		Dec. 31, 2020	Dec. 31, 2019	Mar. 01, 2019
Debt Instrument [Line Items]
Long-term Debt	[1]	$ 1,806.4	$ 1,622.6
Unamortized Debt Issuance Costs		(9.5)	(8.8)
Total Long-Term Debt		1,796.9	1,613.8
Less: Due Within One Year		203.7	212.9
Net Long-Term Debt		$ 1,593.2	1,400.9
First Mortgage Bonds - 5.28% Series Due 2020 [Member]
Debt Instrument [Line Items]
Interest Rate		5.28%
Long-term Debt		$ 0.0	35.0
First Mortgage Bonds - 2.80% Series Due 2020 [Member]
Debt Instrument [Line Items]
Interest Rate		2.80%
Long-term Debt		$ 0.0	40.0
First Mortgage Bonds - 4.85% Series Due 2021 [Member]
Debt Instrument [Line Items]
Interest Rate		4.85%
Long-term Debt		$ 15.0	15.0
First Mortgage Bonds - 3.02% Series Due 2021 [Member]
Debt Instrument [Line Items]
Interest Rate		3.02%
Long-term Debt		$ 60.0	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%		2.50%
Long-term Debt		$ 46.0	0.0
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 - 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%		3.30%
Long-term Debt		$ 94.0	0.0
Variable Demand Revenue Refunding Bonds Series 1997 A Due 2020 [Member]
Debt Instrument [Line Items]
Long-term Debt		0.0	13.5
Unsecured Term Loan Variable Rate Due 2020 [Member]
Debt Instrument [Line Items]
Long-term Debt		$ 0.0	110.0
Debt Instrument, Description of Variable Rate Basis		LIBOR
Debt Instrument, Basis Spread on Variable Rate		1.025%
ALLETE Clean Energy Unsecured Term Loan Variable Rate Due 2021
Debt Instrument [Line Items]
Long-term Debt		$ 65.0	0.0
Unsecured Term Loan Variable Rate Due 2021
Debt Instrument [Line Items]
Long-term Debt		$ 40.0	0.0
Armenia Mountain Senior Secured Notes 3.26% Due 2024 [Member]
Debt Instrument [Line Items]
Interest Rate		3.26%
Long-term Debt		$ 38.6	47.8
Unsecured Senior Notes 2.65% Due 2025
Debt Instrument [Line Items]
Interest Rate		2.65%
Long-term Debt		$ 150.0	0.0
Industrial Development Variable Rate Demand Refunding Revenue Bonds Series 2006, Due 2025 [Member]
Debt Instrument [Line Items]
Long-term Debt		$ 27.8	27.8
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
Other Long-Term Debt, 2.97% – 5.75% Due 2020 – 2037 [Member]
Debt Instrument [Line Items]
Long-term Debt		$ 43.0	$ 46.5
Other Long-Term Debt, 2.97% – 5.75% Due 2020 – 2037 [Member] | Minimum [Member]
Debt Instrument [Line Items]
Interest Rate		2.97%
Other Long-Term Debt, 2.97% – 5.75% Due 2020 – 2037 [Member] | Maximum [Member]
Debt Instrument [Line Items]
Interest Rate		5.75%

[1] Excludes unamortized debt issuance costs.