Consolidated Balance Sheet - USD ($) $ in Millions	Sep. 30, 2015	Dec. 31, 2014
Current Assets [Abstract]
Cash and Cash Equivalents	$ 103.0	$ 145.8
Accounts Receivable (Less Allowance of $1.3 and $1.1)	110.7	103.0
Inventories	119.1	80.5
Prepayments and Other	42.5	82.0
Deferred Income Taxes	28.0	7.5
Total Current Assets	403.3	418.8
Property, Plant and Equipment – Net	3,639.1	3,284.8
Regulatory Assets	353.3	357.3
Investment in ATC	126.0	121.1
Other Investments	113.5	114.4
Goodwill and Intangible Assets – Net	212.9	4.8
Other Non-Current Assets	83.7	59.6
Total Assets	4,931.8	4,360.8
Current Liabilities [Abstract]
Accounts Payable	125.0	134.1
Accrued Taxes	34.8	38.7
Accrued Interest	14.5	18.0
Long-Term Debt Due Within One Year	49.1	100.7
Notes Payable	0.0	3.7
Other	94.3	120.8
Total Current Liabilities	317.7	416.0
Long-Term Debt	1,549.0	1,272.8
Deferred Income Taxes	593.6	510.7
Regulatory Liabilities	105.8	94.2
Defined Benefit Pension and Other Postretirement Benefit Plans	189.6	190.9
Other Non-Current Liabilities	352.7	265.0
Total Liabilities	$ 3,108.4	$ 2,749.6
Commitments, Guarantees and Contingencies (Note 15)
Equity [Abstract]
Common Stock Without Par Value, 80.0 Shares Authorized, 49.0 and 45.9 Shares Outstanding	$ 1,264.9	$ 1,107.6
Unearned ESOP Shares	(1.4)	(7.2)
Accumulated Other Comprehensive Loss	(20.6)	(21.1)
Retained Earnings	578.8	530.1
Total ALLETE Equity	1,821.7	1,609.4
Non-Controlling Interest in Subsidiaries	1.7	1.8
Total Equity	1,823.4	1,611.2
Total Liabilities and Equity	$ 4,931.8	$ 4,360.8