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Business Segments (Details) (USD $)
In Millions, unless otherwise specified
 3 Months Ended 12 Months Ended
Dec. 31, 2012
Sep. 30, 2012
Jun. 30, 2012
Mar. 31, 2012
Dec. 31, 2011
Sep. 30, 2011
Jun. 30, 2011
Mar. 31, 2011
Dec. 31, 2012
Dec. 31, 2011
Dec. 31, 2010
Business Segments [Line Items]                      
Operating Revenue $ 256.0 $ 248.8 $ 216.4 $ 240.0 $ 239.2 $ 226.9 $ 219.9 $ 242.2 $ 961.2 $ 928.2 $ 907.0
Fuel and Purchased Power Expense                 308.7 306.6 325.1
Operating and Maintenance Expense                 397.1 381.2 365.6
Depreciation Expense                 100.2 90.4 80.5
Operating Income (Loss) 47.9 45.6 23.3 38.4 34.2 38.9 26.1 50.8 155.2 150.0 135.8
Interest Expense                 (45.5) (43.6) (39.2)
Equity Earnings in ATC                 19.4 18.4 17.9
Other Income                 6.0 4.4 4.6
Income (Loss) Before Non-Controlling Interest and Income Taxes                 135.1 129.2 119.1
Income Tax Expense (Benefit)                 38.0 35.6 44.3
Net Income (Loss)                 97.1 93.6 74.8
Less: Non-Controlling Interest in Subsidiaries                 0 (0.2) (0.5)
Net Income (Loss) Attributable to ALLETE 28.9 29.4 14.4 24.4 19.1 20.5 17.0 37.2 97.1 93.8 75.3
Total Assets 3,253.4       2,876.0       3,253.4 2,876.0 2,609.1
Capital Additions                 432.2 246.8 260.0
Regulated Operations [Member]
                      
Business Segments [Line Items]                      
Operating Revenue                 874.4 851.9 835.5
Fuel and Purchased Power Expense                 308.7 306.6 325.1
Operating and Maintenance Expense                 310.0 301.5 292.3
Depreciation Expense                 93.9 85.4 76.1
Operating Income (Loss)                 161.8 158.4 142.0
Interest Expense                 (39.8) (35.8) (32.3)
Equity Earnings in ATC                 19.4 18.4 17.9
Other Income                 5.1 2.6 3.8
Income (Loss) Before Non-Controlling Interest and Income Taxes                 146.5 143.6 131.4
Income Tax Expense (Benefit)                 50.4 43.2 51.6
Net Income (Loss)                 96.1 100.4 79.8
Less: Non-Controlling Interest in Subsidiaries                 0 0 0
Net Income (Loss) Attributable to ALLETE                 96.1 100.4 79.8
Total Assets 2,962.4       2,579.8       2,962.4 2,579.8 2,375.4
Capital Additions                 418.2 228.0 256.4
Investments and Other [Member]
                      
Business Segments [Line Items]                      
Number of acres of land in Minnesota (in acres) 6,100               6,100    
Operating Revenue                 86.8 76.3 71.5
Fuel and Purchased Power Expense                 0 0 0
Operating and Maintenance Expense                 87.1 79.7 73.3
Depreciation Expense                 6.3 5.0 4.4
Operating Income (Loss)                 (6.6) (8.4) (6.2)
Interest Expense                 (5.7) (7.8) (6.9)
Equity Earnings in ATC                 0 0 0
Other Income                 0.9 1.8 0.8
Income (Loss) Before Non-Controlling Interest and Income Taxes                 (11.4) (14.4) (12.3)
Income Tax Expense (Benefit)                 (12.4) (7.6) (7.3)
Net Income (Loss)                 1.0 (6.8) (5.0)
Less: Non-Controlling Interest in Subsidiaries                 0 (0.2) (0.5)
Net Income (Loss) Attributable to ALLETE                 1.0 (6.6) (4.5)
Total Assets 291.0       296.2       291.0 296.2 233.7
Capital Additions                 $ 14.0 $ 18.8 $ 3.6