Consolidated Balance Sheet - USD ($) $ in Millions	Jun. 30, 2021	Dec. 31, 2020
Current Assets [Abstract]
Cash and Cash Equivalents	$ 62.5	$ 44.3
Accounts Receivable (Less Allowance of $2.7 and $2.5)	105.2	111.9
Inventories – Net	88.1	74.2
Prepayments and Other	23.5	24.5
Total Current Assets	279.3	254.9
Property, Plant and Equipment – Net	5,021.6	4,840.8
Regulatory Assets	491.9	480.9
Equity Investments	317.8	301.2
Other Non-Current Assets	182.7	206.8
Total Assets	6,293.3	6,084.6
Current Liabilities [Abstract]
Accounts Payable	100.0	110.0
Accrued Taxes	53.0	59.4
Accrued Interest	19.5	19.8
Long-Term Debt Due Within One Year	362.6	203.7
Other	60.4	66.7
Total Current Liabilities	595.5	459.6
Long-Term Debt	1,664.6	1,593.2
Deferred Income Taxes	188.7	195.7
Regulatory Liabilities	514.9	524.8
Defined Benefit Pension and Other Postretirement Benefit Plans	210.7	225.8
Other Non-Current Liabilities	277.0	285.3
Total Liabilities	3,451.4	3,284.4
Commitments, Guarantees and Contingencies (Note 6)
Equity [Abstract]
Common Stock Without Par Value, 80.0 Shares Authorized, 52.3 and 52.1 Shares Issued and Outstanding	1,474.1	1,460.9
Accumulated Other Comprehensive Loss	(30.3)	(31.1)
Retained Earnings	878.8	864.8
Total ALLETE Equity	2,322.6	2,294.6
Non-Controlling Interest in Subsidiaries	519.3	505.6
Total Equity	2,841.9	2,800.2
Total Liabilities and Equity	$ 6,293.3	$ 6,084.6