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Business Segments (Details) (USD $)
In Millions, unless otherwise specified
 3 Months Ended 12 Months Ended
Dec. 31, 2011
Sep. 30, 2011
Jun. 30, 2011
Mar. 31, 2011
Dec. 31, 2010
Sep. 30, 2010
Jun. 30, 2010
Mar. 31, 2010
Dec. 31, 2011
Dec. 31, 2010
Dec. 31, 2009
Business Segments [Abstract]                      
Number of acres of land available-for-sale (in acres) 5,500               5,500    
Business Segments [Line Items]                      
Operating and Maintenance Expense                 $ 381.2 $ 365.6 $ 308.9
Depreciation Expense                 90.4 80.5 64.7
Operating Income 34.2 38.9 26.1 50.8 22.7 35.3 31.7 46.1 150.0 135.8 106.0
Interest Expense                 (43.6) (39.2) (33.8)
Equity Earnings in ATC                 18.4 17.9 17.5
Other Income (Expense)                 4.4 4.6 1.8
Income (Loss) Before Non-Controlling Interest and Income Taxes                 129.2 119.1 91.5
Income Tax Expense (Benefit)                 35.6 44.3 30.8
Net Income (Loss)                 93.6 74.8 60.7
Less: Non-Controlling Interest in Subsidiaries                 (0.2) (0.5) (0.3)
Net Income (Loss) Attributable to ALLETE 19.1 20.5 17.0 37.2 13.3 19.6 19.4 23.0 93.8 75.3 61.0
Total Assets 2,876.0       2,609.1       2,876.0 2,609.1  
Consolidated [Member]
                      
Business Segments [Line Items]                      
Operating Revenue                     766.7
Prior Year Rate Refunds                     (7.6)
Operating Revenue                 928.2 907.0 759.1
Total Operating Revenue                 928.2 907.0 759.1
Fuel and Purchased Power Expense                 306.6 325.1 279.5
Operating and Maintenance Expense                 381.2 365.6 308.9
Depreciation Expense                 90.4 80.5 64.7
Operating Income                 150.0 135.8 106.0
Interest Expense                 (43.6) (39.2) (33.8)
Equity Earnings in ATC                 18.4 17.9 17.5
Other Income (Expense)                 4.4 4.6 1.8
Income (Loss) Before Non-Controlling Interest and Income Taxes                 129.2 119.1 91.5
Income Tax Expense (Benefit)                 35.6 44.3 30.8
Net Income (Loss)                 93.6 74.8 60.7
Less: Non-Controlling Interest in Subsidiaries                 (0.2) (0.5) (0.3)
Net Income (Loss) Attributable to ALLETE                 93.8 75.3 61.0
Total Assets 2,876.0       2,609.1       2,876.0 2,609.1 2,393.1
Capital Additions                 246.8 260.0 303.7
Regulated Operations [Member]
                      
Business Segments [Line Items]                      
Operating Revenue                     689.4
Prior Year Rate Refunds                     (7.6)
Operating Revenue                 851.9 835.5 681.8
Total Operating Revenue                 851.9 835.5 681.8
Fuel and Purchased Power Expense                 306.6 325.1 279.5
Operating and Maintenance Expense                 301.5 292.3 235.8
Depreciation Expense                 85.4 76.1 60.2
Operating Income                 158.4 142.0 106.3
Interest Expense                 (35.8) (32.3) (28.3)
Equity Earnings in ATC                 18.4 17.9 17.5
Other Income (Expense)                 2.6 3.8 5.8
Income (Loss) Before Non-Controlling Interest and Income Taxes                 143.6 131.4 101.3
Income Tax Expense (Benefit)                 43.2 51.6 35.4
Net Income (Loss)                 100.4 79.8 65.9
Less: Non-Controlling Interest in Subsidiaries                 0 0 0
Net Income (Loss) Attributable to ALLETE                 100.4 79.8 65.9
Total Assets 2,579.8       2,375.4       2,579.8 2,375.4 2,184.0
Capital Additions                 228.0 256.4 299.2
Investments and Other [Member]
                      
Business Segments [Line Items]                      
Operating Revenue                     77.3
Prior Year Rate Refunds                     0
Operating Revenue                 76.3 71.5 77.3
Total Operating Revenue                 76.3 71.5 77.3
Fuel and Purchased Power Expense                 0 0 0
Operating and Maintenance Expense                 79.7 73.3 73.1
Depreciation Expense                 5.0 4.4 4.5
Operating Income                 (8.4) (6.2) (0.3)
Interest Expense                 (7.8) (6.9) (5.5)
Equity Earnings in ATC                 0 0 0
Other Income (Expense)                 1.8 0.8 (4.0)
Income (Loss) Before Non-Controlling Interest and Income Taxes                 (14.4) (12.3) (9.8)
Income Tax Expense (Benefit)                 (7.6) (7.3) (4.6)
Net Income (Loss)                 (6.8) (5.0) (5.2)
Less: Non-Controlling Interest in Subsidiaries                 (0.2) (0.5) (0.3)
Net Income (Loss) Attributable to ALLETE                 (6.6) (4.5) (4.9)
Total Assets 296.2       233.7       296.2 233.7 209.1
Capital Additions                 $ 18.8 $ 3.6 $ 4.5