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Business Segments (Details) (USD $)
In Millions, unless otherwise specified
 3 Months Ended 6 Months Ended
Jun. 30, 2013
Jun. 30, 2012
Jun. 30, 2013
Jun. 30, 2012
Dec. 31, 2012
Business Segments [Line Items]          
Operating Revenue $ 235.6 $ 216.4 $ 499.4 $ 456.4  
Fuel and Purchased Power Expense 78.7 72.1 165.2 149.2  
Operating and Maintenance Expense 103.8 96.2 208.5 196.1  
Depreciation Expense 28.7 24.8 56.9 49.4  
Operating Income (Loss) 24.4 23.3 68.8 61.7  
Interest Expense (12.8) (10.1) (25.1) (21.1)  
Equity Earnings in ATC 5.0 4.8 10.2 9.4  
Other Income (Expense) 1.5 1.2 4.2 1.9  
Income (Loss) Before Income Taxes 18.1 19.2 58.1 51.9  
Income Tax Expense (Benefit) 4.1 4.8 11.6 13.1  
Net Income 14.0 14.4 46.5 38.8  
Total Assets 3,350.9 2,962.2 3,350.9 2,962.2 3,253.4
Property, Plant and Equipment - Net 2,397.2 2,177.5 2,397.2 2,177.5 2,347.6
Accumulated Depreciation 1,202.8 1,124.5 1,202.8 1,124.5  
Capital Additions     100.2 235.1  
Regulated Operations [Member]
          
Business Segments [Line Items]          
Operating Revenue 215.8 197.0 457.2 415.6  
Fuel and Purchased Power Expense 78.7 72.1 165.2 149.2  
Operating and Maintenance Expense 82.8 76.1 165.0 154.2  
Depreciation Expense 27.1 23.4 53.9 46.6  
Operating Income (Loss) 27.2 25.4 73.1 65.6  
Interest Expense (10.4) (9.9) (21.1) (19.5)  
Equity Earnings in ATC 5.0 4.8 10.2 9.4  
Other Income (Expense) 1.1 1.2 2.2 2.0  
Income (Loss) Before Income Taxes 22.9 21.5 64.4 57.5  
Income Tax Expense (Benefit) 6.6 7.1 16.0 18.7  
Net Income 16.3 14.4 48.4 38.8  
Total Assets 2,997.0 2,755.4 2,997.0 2,755.4  
Property, Plant and Equipment - Net 2,329.6 2,119.9 2,329.6 2,119.9  
Accumulated Depreciation 1,143.2 1,070.1 1,143.2 1,070.1  
Capital Additions     97.2 232.3  
Investments and Other [Member]
          
Business Segments [Line Items]          
Land in Minnesota (in acres) 6,000   6,000    
Operating Revenue 19.8 19.4 42.2 40.8  
Fuel and Purchased Power Expense 0 0 0 0  
Operating and Maintenance Expense 21.0 20.1 43.5 41.9  
Depreciation Expense 1.6 1.4 3.0 2.8  
Operating Income (Loss) (2.8) (2.1) (4.3) (3.9)  
Interest Expense (2.4) (0.2) (4.0) (1.6)  
Equity Earnings in ATC 0 0 0 0  
Other Income (Expense) 0.4 0 2.0 (0.1)  
Income (Loss) Before Income Taxes (4.8) (2.3) (6.3) (5.6)  
Income Tax Expense (Benefit) (2.5) (2.3) (4.4) (5.6)  
Net Income (2.3) 0 (1.9) 0  
Total Assets 353.9 206.8 353.9 206.8  
Property, Plant and Equipment - Net 67.6 57.6 67.6 57.6  
Accumulated Depreciation 59.6 54.4 59.6 54.4  
Capital Additions     $ 3.0 $ 2.8