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Business Segments (Details) (USD $)
In Millions, unless otherwise specified
 3 Months Ended 9 Months Ended
Sep. 30, 2012
Sep. 30, 2011
Sep. 30, 2012
Sep. 30, 2011
Dec. 31, 2011
Business Segments [Line Items]          
Operating Revenue $ 248.8 $ 226.9 $ 705.2 $ 689.0  
Fuel and Purchased Power Expense 79.5 74.8 228.7 229.8  
Operating and Maintenance Expense 98.7 90.5 294.8 276.3  
Depreciation Expense 25.0 22.7 74.4 67.1  
Operating Income (Loss) 45.6 38.9 107.3 115.8  
Interest Expense (12.3) (10.9) (33.4) (32.6)  
Equity Earnings in ATC 4.9 4.7 14.3 13.7  
Other Income (Expense) 1.5 0.5 3.4 2.3  
Income (Loss) Before Non-Controlling Interest and Income Taxes 39.7 33.2 91.6 99.2  
Income Tax Expense (Benefit) 10.3 12.7 23.4 24.7  
Net Income (Loss) 29.4 20.5 68.2 74.5  
Less: Non-Controlling Interest in Subsidiaries 0 0 0 (0.2)  
Net Income (Loss) Attributable to ALLETE 29.4 20.5 68.2 74.7  
Total Assets 3,138.1 2,754.4 3,138.1 2,754.4 2,876.0
Property, Plant and Equipment - Net 2,239.9 1,902.1 2,239.9 1,902.1 1,982.7
Accumulated Depreciation 1,146.7 1,079.0 1,146.7 1,079.0  
Capital Additions     318.3 143.5  
Regulated Operations [Member]
          
Business Segments [Line Items]          
Operating Revenue 226.4 207.4 642.0 632.2  
Fuel and Purchased Power Expense 79.5 74.8 228.7 229.8  
Operating and Maintenance Expense 76.4 70.4 230.6 218.8  
Depreciation Expense 23.5 21.4 70.1 63.5  
Operating Income (Loss) 47.0 40.8 112.6 120.1  
Interest Expense (10.2) (9.2) (29.7) (26.9)  
Equity Earnings in ATC 4.9 4.7 14.3 13.7  
Other Income (Expense) 1.5 0.6 3.5 1.8  
Income (Loss) Before Non-Controlling Interest and Income Taxes 43.2 36.9 100.7 108.7  
Income Tax Expense (Benefit) 13.9 13.1 32.6 28.2  
Net Income (Loss) 29.3 23.8 68.1 80.5  
Less: Non-Controlling Interest in Subsidiaries 0 0 0 0  
Net Income (Loss) Attributable to ALLETE 29.3 23.8 68.1 80.5  
Total Assets 2,830.9 2,436.0 2,830.9 2,436.0  
Property, Plant and Equipment - Net 2,180.8 1,847.1 2,180.8 1,847.1  
Accumulated Depreciation 1,091.5 1,028.6 1,091.5 1,028.6  
Capital Additions     312.6 128.4  
Investments and Other [Member]
          
Business Segments [Line Items]          
Land available-for-sale in Minnesota (in acres) 5,500   5,500    
Operating Revenue 22.4 19.5 63.2 56.8  
Fuel and Purchased Power Expense 0 0 0 0  
Operating and Maintenance Expense 22.3 20.1 64.2 57.5  
Depreciation Expense 1.5 1.3 4.3 3.6  
Operating Income (Loss) (1.4) (1.9) (5.3) (4.3)  
Interest Expense (2.1) (1.7) (3.7) (5.7)  
Equity Earnings in ATC 0 0 0 0  
Other Income (Expense) 0 (0.1) (0.1) 0.5  
Income (Loss) Before Non-Controlling Interest and Income Taxes (3.5) (3.7) (9.1) (9.5)  
Income Tax Expense (Benefit) (3.6) (0.4) (9.2) (3.5)  
Net Income (Loss) 0.1 (3.3) 0.1 (6.0)  
Less: Non-Controlling Interest in Subsidiaries 0 0 0 (0.2)  
Net Income (Loss) Attributable to ALLETE 0.1 (3.3) 0.1 (5.8)  
Total Assets 307.2 318.4 307.2 318.4  
Property, Plant and Equipment - Net 59.1 55.0 59.1 55.0  
Accumulated Depreciation 55.2 50.4 55.2 50.4  
Capital Additions     $ 5.7 $ 15.1