Consolidated Balance Sheet - USD ($) $ in Millions	Dec. 31, 2019	Dec. 31, 2018
Current Assets [Abstract]
Cash and Cash Equivalents	$ 69.3	$ 69.1
Accounts Receivable (Less Allowance of $0.9 and $1.7)	96.4	144.4
Inventories – Net	72.8	86.7
Prepayments and Other	31.0	34.1
Total Current Assets	269.5	334.3
Property, Plant and Equipment – Net	4,377.0	3,904.4
Regulatory Assets	420.5	389.5
Equity Investments	197.6	161.1
Goodwill and Intangible Assets – Net	1.0	223.3
Other Non-Current Assets	217.2	152.4
Total Assets	5,482.8	5,165.0
Current Liabilities [Abstract]
Accounts Payable	165.2	149.8
Accrued Taxes	50.8	51.4
Accrued Interest	18.1	17.9
Long-Term Debt Due Within One Year	212.9	57.5
Other	60.4	128.5
Total Current Liabilities	507.4	405.1
Long-Term Debt	1,400.9	1,428.5
Deferred Income Taxes	212.8	223.6
Regulatory Liabilities	560.3	512.1
Defined Benefit Pension and Other Postretirement Benefit Plans	172.8	177.3
Other Non-Current Liabilities	293.0	262.6
Total Liabilities	3,147.2	3,009.2
Commitments, Guarantees and Contingencies (Note 9)
ALLETE's Equity [Abstract]
Common Stock Without Par Value, 80.0 Shares Authorized, 51.7 and 51.5 Shares Issued and Outstanding	1,436.7	1,428.5
Accumulated Other Comprehensive Loss	(23.6)	(27.3)
Retained Earnings	818.8	754.6
Total ALLETE Equity	2,231.9	2,155.8
Non-controlling Interest in Subsidiaries	103.7	0.0
Total Equity	2,335.6	2,155.8
Total Liabilities and Equity	$ 5,482.8	$ 5,165.0