Business Segments (Details) (USD $) In Millions	3 Months Ended		6 Months Ended
	Jun. 30, 2011	Jun. 30, 2010	Jun. 30, 2011	Jun. 30, 2010	Dec. 31, 2010
Segment Reporting Information [Line Items]
Operating Revenue	$ 219.9	$ 211.2	$ 462.1	$ 444.8
Fuel and Purchased Power	76.0	74.3	155.0	154.1
Operating and Maintenance	95.7	85.4	185.8	173.1
Depreciation	22.1	19.8	44.4	39.8
Total Operating Expenses	193.8	179.5	385.2	367.0
Operating Income	26.1	31.7	76.9	77.8
Interest Expense	(11.0)	(9.5)	(21.7)	(18.4)
Equity Earnings in ATC	4.6	4.4	9.0	8.9
Other Income	1.0	2.2	1.8	3.2
Income (Loss) Before Non-Controlling Interest and Income Taxes	20.7	28.8	66.0	71.5
Income Tax Expense (Benefit)	3.8	9.4	12.0	29.3
Net Income	16.9	19.4	54.0	42.2
Less: Non-Controlling Interest in Subsidiaries	(0.1)	0	(0.2)	(0.2)
Net Income (Loss) Attributable to ALLETE	17.0	19.4	54.2	42.4
Total Assets	2,645.5	2,447.8	2,645.5	2,447.8	2,609.1
Property, Plant and Equipment - Net	1,861.1	1,671.7	1,861.1	1,671.7	1,805.6
Accumulated Depreciation	1,062.6	1,007.5	1,062.6	1,007.5
Capital Additions			79.7	85.1
Regulated Operatons[Member]
Segment Reporting Information [Line Items]
Operating Revenue	201.8	194.1	424.8	410.2
Fuel and Purchased Power	76.0	74.3	155.0	154.1
Operating and Maintenance	77.2	69.3	148.4	139.1
Depreciation	20.9	18.7	42.1	37.7
Operating Income	27.7	31.8	79.3	79.3
Interest Expense	(9.1)	(7.7)	(17.7)	(15.3)
Equity Earnings in ATC	4.6	4.4	9.0	8.9
Other Income	0.6	1.1	1.2	2.3
Income (Loss) Before Non-Controlling Interest and Income Taxes	23.8	29.6	71.8	75.2
Income Tax Expense (Benefit)	5.5	11.4	15.1	32.1
Net Income	18.3	18.2	56.7	43.1
Less: Non-Controlling Interest in Subsidiaries	0	0	0	0
Net Income (Loss) Attributable to ALLETE	18.3	18.2	56.7	43.1
Total Assets	2,381.6	2,211.2	2,381.6	2,211.2
Property, Plant and Equipment - Net	1,808.9	1,627.4	1,808.9	1,627.4
Accumulated Depreciation	1,011.3	959.4	1,011.3	959.4
Capital Additions			69.3	85.0
Investment and Other [Member]
Segment Reporting Information [Line Items]
Operating Revenue	18.1	17.1	37.3	34.6
Fuel and Purchased Power	0	0	0	0
Operating and Maintenance	18.5	16.1	37.4	34.0
Depreciation	1.2	1.1	2.3	2.1
Operating Income	(1.6)	(0.1)	(2.4)	(1.5)
Interest Expense	(1.9)	(1.8)	(4.0)	(3.1)
Equity Earnings in ATC	0	0	0	0
Other Income	0.4	1.1	0.6	0.9
Income (Loss) Before Non-Controlling Interest and Income Taxes	(3.1)	(0.8)	(5.8)	(3.7)
Income Tax Expense (Benefit)	(1.7)	(2.0)	(3.1)	(2.8)
Net Income	(1.4)	1.2	(2.7)	(0.9)
Less: Non-Controlling Interest in Subsidiaries	(0.1)	0	(0.2)	(0.2)
Net Income (Loss) Attributable to ALLETE	(1.3)	1.2	(2.5)	(0.7)
Total Assets	263.9	236.6	263.9	236.6
Property, Plant and Equipment - Net	52.2	44.3	52.2	44.3
Accumulated Depreciation	51.3	48.1	51.3	48.1
Capital Additions			$ 10.4	$ 0.1