Consolidated Balance Sheet - USD ($) $ in Millions	Mar. 31, 2022	Dec. 31, 2021
Current Assets [Abstract]
Cash and Cash Equivalents	$ 60.1	$ 45.1
Accounts Receivable (Less Allowance of $1.9 and $1.8)	118.4	123.7
Inventories – Net	196.6	97.7
Prepayments and Other	105.4	24.8
Total Current Assets	480.5	291.3
Property, Plant and Equipment – Net	5,078.3	5,100.2
Regulatory Assets	464.2	511.8
Equity Investments	320.6	318.0
Other Non-Current Assets	212.4	213.7
Total Assets	6,556.0	6,435.0
Current Liabilities [Abstract]
Accounts Payable	75.9	111.0
Accrued Taxes	78.5	65.1
Accrued Interest	15.3	20.1
Long-Term Debt Due Within One Year	278.1	214.2
Other	108.8	133.0
Total Current Liabilities	556.6	543.4
Long-Term Debt	1,669.4	1,763.2
Deferred Income Taxes	188.3	185.7
Regulatory Liabilities	533.4	536.1
Defined Benefit Pension and Other Postretirement Benefit Plans	177.2	179.5
Other Non-Current Liabilities	287.6	280.8
Total Liabilities	3,412.5	3,488.7
Commitments, Guarantees and Contingencies (Note 6)
Equity [Abstract]
Common Stock Without Par Value, 80.0 Shares Authorized, 53.3 and 53.2 Shares Issued and Outstanding	1,541.3	1,536.7
Accumulated Other Comprehensive Loss	(24.0)	(23.8)
Retained Earnings	932.0	900.2
Total ALLETE Equity	2,449.3	2,413.1
Non-Controlling Interest in Subsidiaries	694.2	533.2
Total Equity	3,143.5	2,946.3
Total Liabilities and Equity	$ 6,556.0	$ 6,435.0