Business Segments (Details) (USD $) In Millions, unless otherwise specified	3 Months Ended		6 Months Ended
	Jun. 30, 2012	Jun. 30, 2011	Jun. 30, 2012	Jun. 30, 2011	Dec. 31, 2011
Business Segments [Line Items]
Operating Revenue	$ 216.4	$ 219.9	$ 456.4	$ 462.1
Fuel and Purchased Power Expense	72.1	76.0	149.2	155.0
Operating and Maintenance Expense	96.2	95.7	196.1	185.8
Depreciation Expense	24.8	22.1	49.4	44.4
Operating Income (Loss)	23.3	26.1	61.7	76.9
Interest Expense	(10.1)	(11.0)	(21.1)	(21.7)
Equity Earnings in ATC	4.8	4.6	9.4	9.0
Other Income	1.2	1.0	1.9	1.8
Income (Loss) Before Non-Controlling Interest and Income Taxes	19.2	20.7	51.9	66.0
Income Tax Expense (Benefit)	4.8	3.8	13.1	12.0
Net Income (Loss)	14.4	16.9	38.8	54.0
Less: Non-Controlling Interest in Subsidiaries	0	(0.1)	0	(0.2)
Net Income Attributable to ALLETE	14.4	17.0	38.8	54.2
Total Assets	2,962.2	2,645.5	2,962.2	2,645.5	2,876.0
Property, Plant and Equipment - Net	2,177.5	1,861.1	2,177.5	1,861.1	1,982.7
Accumulated Depreciation	1,124.5	1,062.6	1,124.5	1,062.6
Capital Additions			235.1	79.7
Regulated Operations [Member]
Business Segments [Line Items]
Operating Revenue	197.0	201.8	415.6	424.8
Fuel and Purchased Power Expense	72.1	76.0	149.2	155.0
Operating and Maintenance Expense	76.1	77.2	154.2	148.4
Depreciation Expense	23.4	20.9	46.6	42.1
Operating Income (Loss)	25.4	27.7	65.6	79.3
Interest Expense	(9.9)	(9.1)	(19.5)	(17.7)
Equity Earnings in ATC	4.8	4.6	9.4	9.0
Other Income	1.2	0.6	2.0	1.2
Income (Loss) Before Non-Controlling Interest and Income Taxes	21.5	23.8	57.5	71.8
Income Tax Expense (Benefit)	7.1	5.5	18.7	15.1
Net Income (Loss)	14.4	18.3	38.8	56.7
Less: Non-Controlling Interest in Subsidiaries	0	0	0	0
Net Income Attributable to ALLETE	14.4	18.3	38.8	56.7
Total Assets	2,755.4	2,381.6	2,755.4	2,381.6
Property, Plant and Equipment - Net	2,119.9	1,808.9	2,119.9	1,808.9
Accumulated Depreciation	1,070.1	1,011.3	1,070.1	1,011.3
Capital Additions			232.3	69.3
Investments and Other [Member]
Business Segments [Line Items]
Land available-for-sale in Minnesota (in acres)	5,500		5,500
Operating Revenue	19.4	18.1	40.8	37.3
Fuel and Purchased Power Expense	0	0	0	0
Operating and Maintenance Expense	20.1	18.5	41.9	37.4
Depreciation Expense	1.4	1.2	2.8	2.3
Operating Income (Loss)	(2.1)	(1.6)	(3.9)	(2.4)
Interest Expense	(0.2)	(1.9)	(1.6)	(4.0)
Equity Earnings in ATC	0	0	0	0
Other Income	0	0.4	(0.1)	0.6
Income (Loss) Before Non-Controlling Interest and Income Taxes	(2.3)	(3.1)	(5.6)	(5.8)
Income Tax Expense (Benefit)	(2.3)	(1.7)	(5.6)	(3.1)
Net Income (Loss)	0	(1.4)	0	(2.7)
Less: Non-Controlling Interest in Subsidiaries	0	(0.1)	0	(0.2)
Net Income Attributable to ALLETE	0	(1.3)	0	(2.5)
Total Assets	206.8	263.9	206.8	263.9
Property, Plant and Equipment - Net	57.6	52.2	57.6	52.2
Accumulated Depreciation	54.4	51.3	54.4	51.3
Capital Additions			$ 2.8	$ 10.4