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Property, Plant and Equipment (Details) (USD $)
In Millions, unless otherwise specified
 12 Months Ended
Dec. 31, 2011
Dec. 31, 2010
Property, Plant and Equipment [Line Items]    
Property, Plant and Equipment - Net $ 1,982.7 $ 1,805.6
Asset Retirement Obligation [Roll Forward]    
Beginning Obligation 50.3 44.6
Accretion Expense 6.4 2.9
Additional Liabilities Incurred 0.3 2.8
Ending Obligation 57.0 50.3
Regulated Utiity [Member]
    
Property, Plant and Equipment [Line Items]    
Segment Property, Plant and Equipment 2,794.8 2,649.2
Construction Work in Progress 155.0 86.6
Accumulated Depreciation (1,024.6) (975.8)
Segment Property, Plant and Equipment - Net 1,925.2 1,760.0
Estimated Useful Life - Generation 4 to 35 years  
Estimated Useful Life - Transmission 42 to 61 years  
Estimated Useful Life - Distribution 14 to 65 years  
Unregulated Non-Rate Base Energy Operations and Other Plant [Member]
    
Property, Plant and Equipment [Line Items]    
Segment Property, Plant and Equipment 106.4 88.4
Construction Work in Progress 2.3 4.5
Accumulated Depreciation (51.4) (48.0)
Segment Property, Plant and Equipment - Net 57.3 44.9
Other Plant - Net $ 0.2 $ 0.7
Estimated Useful Life - Non-Rate Base Operations 3 to 61 years  
Estimated Useful Life - Other Plant 5 to 25 years