Consolidated Balance Sheet (USD $) In Millions, unless otherwise specified	Mar. 31, 2015	Dec. 31, 2014
Current Assets [Abstract]
Cash and Cash Equivalents	$ 72.1	$ 145.8
Accounts Receivable (Less Allowance of $1.1 and $1.1)	111.8	103.0
Inventories	102.7	80.5
Prepayments and Other	88.5	82.0
Deferred Income Taxes	14.9	7.5
Total Current Assets	390.0	418.8
Property, Plant and Equipment - Net	3,319.2	3,284.8
Regulatory Assets	358.0	357.3
Investment in ATC	122.3	121.1
Other Investments	114.7	114.4
Goodwill and Intangible Assets – Net	214.3	4.8
Other Non-Current Assets	61.6	59.6
Total Assets	4,580.1	4,360.8
Current Liabilities [Abstract]
Accounts Payable	98.9	134.1
Accrued Taxes	47.4	38.7
Accrued Interest	14.5	18.0
Long-Term Debt Due Within One Year	122.3	100.7
Notes Payable	0.3	3.7
Other	118.6	120.8
Total Current Liabilities	402.0	416.0
Long-Term Debt	1,253.8	1,272.8
Deferred Income Taxes	559.5	510.7
Regulatory Liabilities	101.2	94.2
Defined Benefit Pension and Other Postretirement Benefit Plans	190.4	190.9
Other Non-Current Liabilities	302.3	265.0
Total Liabilities	2,809.2	2,749.6
Commitments, Guarantees and Contingencies (Note 16)
Equity [Abstract]
Common Stock Without Par Value, 80.0 Shares Authorized, 48.8 and 45.9 Shares Outstanding	1,249.7	1,107.6
Unearned ESOP Shares	(5.3)	(7.2)
Accumulated Other Comprehensive Loss	(20.6)	(21.1)
Retained Earnings	545.1	530.1
Total ALLETE Equity	1,768.9	1,609.4
Non-Controlling Interest in Subsidiaries	2.0	1.8
Total Equity	1,770.9	1,611.2
Total Liabilities and Equity	$ 4,580.1	$ 4,360.8