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Property, Plant and Equipment (Details) (USD $)
In Millions, unless otherwise specified
 12 Months Ended
Dec. 31, 2012
Dec. 31, 2011
Dec. 31, 2010
Property, Plant and Equipment [Line Items]      
Total Property, Plant and Equipment - Net $ 2,347.6 $ 1,982.7  
Asset Retirement Obligation [Roll Forward]      
Beginning Obligation 57.0 50.3  
Accretion Expense 3.8 6.4  
Additional Liabilities Incurred 17.1 0.3 2.8
Ending Obligation 77.9 57.0 50.3
Regulated Utiity [Member]
      
Property, Plant and Equipment [Line Items]      
Property, Plant and Equipment 3,232.9 2,794.8  
Construction Work in Progress 151.8 155.0  
Accumulated Depreciation (1,102.8) (1,024.6)  
Property, Plant and Equipment - Net 2,281.9 1,925.2  
Estimated Useful Life - Generation 3 to 35 years    
Estimated Useful Life - Transmission 42 to 61 years    
Estimated Useful Life - Distribution 14 to 65 years    
Non-Rate Base Energy Operations and Other Plant [Member]
      
Property, Plant and Equipment [Line Items]      
Property, Plant and Equipment 118.0 106.4  
Construction Work in Progress 4.2 2.3  
Accumulated Depreciation (56.7) (51.4)  
Property, Plant and Equipment - Net 65.5 57.3  
Other Plant - Net $ 0.2 $ 0.2  
Estimated Useful Life - Non-Rate Base Operations 3 to 61 years    
Estimated Useful Life - Other Plant 5 to 25 years