Short-Term and Long-Term Debt (Details) - USD ($) $ in Millions	Apr. 27, 2023	Jun. 30, 2023	Dec. 31, 2022
Debt Instrument [Line Items]
Short-Term Debt - Principal		$ 106.8	$ 272.7
Short-Term Debt - Unamortized Debt Issuance Costs		(0.1)	(0.1)
Short-Term Debt - Total		106.7	272.6
Long-Term Debt - Principal		1,694.5	1,656.4
Long-Term Debt - Unamortized Debt Issuance Costs		(8.6)	(8.2)
Long-Term Debt - Total		1,685.9	1,648.2
Total Debt - Principal		1,801.3	1,929.1
Total Debt - Unamortized Debt Issuance Costs		(8.7)	(8.3)
Total Debt - Total		1,792.6	1,920.8
Line of Credit [Member]
Debt Instrument [Line Items]
Letters of Credit Outstanding, Amount		17.8	32.8
Line of Credit, Current		$ 29.4	$ 31.3
ALLETE Bonds 4.98% Due April 2033
Debt Instrument [Line Items]
Proceeds from Issuance of First Mortgage Bond	$ 125.0
Interest Rate	4.98%