Consolidated Balance Sheet - USD ($) $ in Millions	Mar. 31, 2020	Dec. 31, 2019
Current Assets [Abstract]
Cash and Cash Equivalents	$ 67.0	$ 69.3
Accounts Receivable (Less Allowance of $1.2 and $0.9)	99.4	96.4
Inventories – Net	79.0	72.8
Prepayments and Other	29.4	31.0
Total Current Assets	274.8	269.5
Property, Plant and Equipment – Net	4,496.3	4,377.0
Regulatory Assets	423.8	420.5
Equity Investments	225.7	197.6
Other Non-Current Assets	198.9	218.2
Total Assets	5,619.5	5,482.8
Current Liabilities [Abstract]
Accounts Payable	164.4	165.2
Accrued Taxes	63.0	50.8
Accrued Interest	15.1	18.1
Long-Term Debt Due Within One Year	323.0	212.9
Other	57.6	60.4
Total Current Liabilities	623.1	507.4
Long-Term Debt	1,399.9	1,400.9
Deferred Income Taxes	207.0	212.8
Regulatory Liabilities	566.4	560.3
Defined Benefit Pension and Other Postretirement Benefit Plans	161.4	172.8
Other Non-Current Liabilities	288.7	293.0
Total Liabilities	3,246.5	3,147.2
Commitments, Guarantees and Contingencies (Note 6)
Shareholders’ Equity [Abstract]
Common Stock Without Par Value, 80.0 Shares Authorized, 51.8 and 51.7 Shares Issued and Outstanding	1,441.7	1,436.7
Accumulated Other Comprehensive Loss	(23.8)	(23.6)
Retained Earnings	853.2	818.8
Total ALLETE Equity	2,271.1	2,231.9
Non-Controlling Interest in Subsidiaries	101.9	103.7
Total Equity	2,373.0	2,335.6
Total Liabilities and Equity	$ 5,619.5	$ 5,482.8