Consolidated Balance Sheet - USD ($) $ in Millions	Sep. 30, 2020	Dec. 31, 2019
Current Assets [Abstract]
Cash and Cash Equivalents	$ 79.0	$ 69.3
Accounts Receivable (Less Allowance of $1.6 and $0.9)	92.0	96.4
Inventories – Net	74.8	72.8
Prepayments and Other	19.3	31.0
Total Current Assets	265.1	269.5
Property, Plant and Equipment – Net	4,697.5	4,377.0
Regulatory Assets	427.9	420.5
Equity Investments	291.8	197.6
Other Non-Current Assets	196.2	218.2
Total Assets	5,878.5	5,482.8
Current Liabilities [Abstract]
Accounts Payable	79.0	165.2
Accrued Taxes	60.0	50.8
Accrued Interest	15.8	18.1
Long-Term Debt Due Within One Year	404.5	212.9
Other	70.7	60.4
Total Current Liabilities	630.0	507.4
Long-Term Debt	1,608.0	1,400.9
Deferred Income Taxes	204.8	212.8
Regulatory Liabilities	547.1	560.3
Defined Benefit Pension and Other Postretirement Benefit Plans	157.5	172.8
Other Non-Current Liabilities	285.5	293.0
Total Liabilities	3,432.9	3,147.2
Commitments, Guarantees and Contingencies (Note 6)
Equity [Abstract]
Common Stock Without Par Value, 80.0 Shares Authorized, 52.0 and 51.7 Shares Issued and Outstanding	1,454.2	1,436.7
Accumulated Other Comprehensive Loss	(23.1)	(23.6)
Retained Earnings	849.9	818.8
Total ALLETE Equity	2,281.0	2,231.9
Non-Controlling Interest in Subsidiaries	164.6	103.7
Total Equity	2,445.6	2,335.6
Total Liabilities and Equity	$ 5,878.5	$ 5,482.8