Consolidated Balance Sheet - USD ($) $ in Millions	Dec. 31, 2020	Dec. 31, 2019
Current Assets [Abstract]
Cash and Cash Equivalents	$ 44.3	$ 69.3
Accounts Receivable (Less Allowance of $2.5 and $0.9)	111.9	96.4
Inventories – Net	74.2	72.8
Prepayments and Other	24.5	31.0
Total Current Assets	254.9	269.5
Property, Plant and Equipment – Net	4,840.8	4,377.0
Regulatory Assets	480.9	420.5
Equity Investments	301.2	197.6
Other Non-Current Assets	206.8	218.2
Total Assets	6,084.6	5,482.8
Current Liabilities [Abstract]
Accounts Payable	110.0	165.2
Accrued Taxes	59.4	50.8
Accrued Interest	19.8	18.1
Long-Term Debt Due Within One Year	203.7	212.9
Other	66.7	60.4
Total Current Liabilities	459.6	507.4
Long-Term Debt	1,593.2	1,400.9
Deferred Income Taxes	195.7	212.8
Regulatory Liabilities	524.8	560.3
Defined Benefit Pension and Other Postretirement Benefit Plans	225.8	172.8
Other Non-Current Liabilities	285.3	293.0
Total Liabilities	3,284.4	3,147.2
Commitments, Guarantees and Contingencies (Note 8)
ALLETE Equity [Abstract]
Common Stock Without Par Value, 80.0 Shares Authorized, 52.1 and 51.7 Shares Issued and Outstanding	1,460.9	1,436.7
Accumulated Other Comprehensive Loss	(31.1)	(23.6)
Retained Earnings	864.8	818.8
Total ALLETE Equity	2,294.6	2,231.9
Non-Controlling Interest in Subsidiaries	505.6	103.7
Total Equity	2,800.2	2,335.6
Total Liabilities and Equity	$ 6,084.6	$ 5,482.8