Short-Term and Long-Term Debt - Schedule of Long-Term Debt (Details) - USD ($) $ in Millions		12 Months Ended
		Dec. 31, 2022	Dec. 31, 2021
Debt Instrument [Line Items]
Long-term Debt	[1]	$ 1,929.1	$ 1,986.4
Unamortized Debt Issuance Costs		(8.3)	(9.0)
Total Long-Term Debt		1,920.8	1,977.4
Less: Due Within One Year		272.6	214.2
Net Long-Term Debt		$ 1,648.2	1,763.2
First Mortgage Bonds - 4.85% Series Due 2021 [Member]
Debt Instrument [Line Items]
Interest Rate		4.85%
First Mortgage Bonds - 3.02% Series Due 2021 [Member]
Debt Instrument [Line Items]
Interest Rate		3.02%
First Mortgage Bonds - 3.40% Series Due 2022 [Member]
Debt Instrument [Line Items]
Interest Rate		3.40%
Long-term Debt		$ 0.0	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	100.0
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
Armenia Mountain Senior Secured Notes 3.26% Due 2024 [Member]
Debt Instrument [Line Items]
Interest Rate		3.26%
Long-term Debt		$ 19.3	29.1
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		$ 13.0	145.0
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 2022
Debt Instrument [Line Items]
Long-term Debt		0.0	110.0
Other Long-Term Debt, 2.16% – 4.58% Due 2022 – 2051 [Member]
Debt Instrument [Line Items]
Long-term Debt		$ 82.0	57.5
Other Long-Term Debt, 2.16% – 4.58% Due 2022 – 2051 [Member] | Weighted Average
Debt Instrument [Line Items]
Interest Rate		4.94%
4.54% Series Due 2032
Debt Instrument [Line Items]
Interest Rate		4.54%
Long-term Debt		$ 75.0	0.0
Unsecured Term Loan Variable Rate Due 2023
Debt Instrument [Line Items]
Long-term Debt		$ 170.0	$ 0.0
March Unsecured Term Loan Due in 2023
Debt Instrument [Line Items]
Debt Instrument, Description of Variable Rate Basis		SOFR
Debt Instrument, Basis Spread on Variable Rate		0.75%

[1] Excludes unamortized debt issuance costs.