Consolidated Balance Sheet - USD ($) $ in Millions	Jun. 30, 2020	Dec. 31, 2019
Current Assets [Abstract]
Cash and Cash Equivalents	$ 25.7	$ 69.3
Accounts Receivable (Less Allowance of $1.3 and $0.9)	85.8	96.4
Inventories – Net	82.0	72.8
Prepayments and Other	29.5	31.0
Total Current Assets	223.0	269.5
Property, Plant and Equipment – Net	4,644.5	4,377.0
Regulatory Assets	424.7	420.5
Equity Investments	267.1	197.6
Other Non-Current Assets	201.1	218.2
Total Assets	5,760.4	5,482.8
Current Liabilities [Abstract]
Accounts Payable	86.1	165.2
Accrued Taxes	48.5	50.8
Accrued Interest	18.0	18.1
Long-Term Debt Due Within One Year	509.7	212.9
Other	65.4	60.4
Total Current Liabilities	727.7	507.4
Long-Term Debt	1,381.0	1,400.9
Deferred Income Taxes	202.3	212.8
Regulatory Liabilities	569.6	560.3
Defined Benefit Pension and Other Postretirement Benefit Plans	159.5	172.8
Other Non-Current Liabilities	288.1	293.0
Total Liabilities	3,328.2	3,147.2
Commitments, Guarantees and Contingencies (Note 6)
Equity [Abstract]
Common Stock Without Par Value, 80.0 Shares Authorized, 51.9 and 51.7 Shares Issued and Outstanding	1,447.7	1,436.7
Accumulated Other Comprehensive Loss	(23.3)	(23.6)
Retained Earnings	841.3	818.8
Total ALLETE Equity	2,265.7	2,231.9
Non-Controlling Interest in Subsidiaries	166.5	103.7
Total Equity	2,432.2	2,335.6
Total Liabilities and Equity	$ 5,760.4	$ 5,482.8