Consolidated Balance Sheet (USD $) In Millions, unless otherwise specified	Mar. 31, 2012	Dec. 31, 2011
Current Assets [Abstract]
Cash and Cash Equivalents	$ 118.2	$ 101.1
Accounts Receivable (Less Allowance of $1.0 and $0.9)	76.7	79.7
Inventories	72.2	69.1
Prepayments and Other	22.9	27.1
Total Current Assets	290.0	277.0
Property, Plant and Equipment - Net	2,002.8	1,982.7
Regulatory Assets	339.3	345.9
Investment in ATC	100.5	98.9
Other Investments	135.7	132.3
Other Non-Current Assets	38.3	39.2
Total Assets	2,906.6	2,876.0
Current Liabilities [Abstract]
Accounts Payable	51.5	71.8
Accrued Taxes	30.8	26.4
Accrued Interest	11.3	12.8
Long-Term Debt Due Within One Year	5.5	5.4
Notes Payable	1.6	1.1
Other	41.8	45.6
Total Current Liabilities	142.5	163.1
Long-Term Debt	856.5	857.9
Deferred Income Taxes	383.4	373.6
Regulatory Liabilities	45.3	43.5
Defined Benefit Pension and Other Postretirement Benefit Plans	254.0	253.5
Other Non-Current Liabilities	111.1	105.1
Total Liabilities	1,792.8	1,796.7
Commitments, Guarantees and Contingencies (Note 13)
ALLETE's Equity [Abstract]
Common Stock Without Par Value, 80 Shares Authorized, 38.1 and 37.5 Shares Outstanding	730.0	705.6
Unearned ESOP Shares	(27.0)	(29.0)
Accumulated Other Comprehensive Loss	(27.5)	(28.9)
Retained Earnings	438.3	431.6
Total Equity	1,113.8	1,079.3
Total Liabilities and Equity	$ 2,906.6	$ 2,876.0