Business Segments (Details) (USD $) In Millions, unless otherwise specified	3 Months Ended
	Mar. 31, 2012	Mar. 31, 2011	Dec. 31, 2011
Business Segments [Line Items]
Operating Revenue	$ 240.0	$ 242.2
Fuel and Purchased Power Expense	77.1	79.0
Operating and Maintenance Expense	99.9	90.1
Depreciation Expense	24.6	22.3
Operating Income (Loss)	38.4	50.8
Interest Expense	(11.0)	(10.7)
Equity Earnings in ATC	4.6	4.4
Other Income (Loss)	0.7	0.8
Income (Loss) Before Non-Controlling Interest and Income Taxes	32.7	45.3
Income Tax Expense (Benefit)	8.3	8.2
Net Income	24.4	37.1
Less: Non-Controlling Interest in Subsidiaries	0	(0.1)
Net Income Attributable to ALLETE	24.4	37.2
Total Assets	2,906.6	2,613.4	2,876.0
Property, Plant and Equipment - Net	2,002.8	1,841.3	1,982.7
Accumulated Depreciation	1,099.0	1,043.4
Capital Additions	39.7	35.9
Regulated Operations [Member]
Business Segments [Line Items]
Operating Revenue	218.6	223.0
Fuel and Purchased Power Expense	77.1	79.0
Operating and Maintenance Expense	78.1	71.2
Depreciation Expense	23.2	21.2
Operating Income (Loss)	40.2	51.6
Interest Expense	(9.6)	(8.6)
Equity Earnings in ATC	4.6	4.4
Other Income (Loss)	0.8	0.6
Income (Loss) Before Non-Controlling Interest and Income Taxes	36.0	48.0
Income Tax Expense (Benefit)	11.6	9.6
Net Income	24.4	38.4
Less: Non-Controlling Interest in Subsidiaries	0	0
Net Income Attributable to ALLETE	24.4	38.4
Total Assets	2,586.4	2,381.8
Property, Plant and Equipment - Net	1,945.6	1,794.6
Accumulated Depreciation	1,045.9	993.1
Capital Additions	38.7	33.0
Investments and Other [Member]
Business Segments [Line Items]
Number of acres of land available-for-sale in Minnesota	5,500
Operating Revenue	21.4	19.2
Fuel and Purchased Power Expense	0	0
Operating and Maintenance Expense	21.8	18.9
Depreciation Expense	1.4	1.1
Operating Income (Loss)	(1.8)	(0.8)
Interest Expense	(1.4)	(2.1)
Equity Earnings in ATC	0	0
Other Income (Loss)	(0.1)	0.2
Income (Loss) Before Non-Controlling Interest and Income Taxes	(3.3)	(2.7)
Income Tax Expense (Benefit)	(3.3)	(1.4)
Net Income	0	(1.3)
Less: Non-Controlling Interest in Subsidiaries	0	(0.1)
Net Income Attributable to ALLETE	0	(1.2)
Total Assets	320.2	231.6
Property, Plant and Equipment - Net	57.2	46.7
Accumulated Depreciation	53.1	50.3
Capital Additions	$ 1.0	$ 2.9