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Regulatory Matters - Regulatory Liabilities (Details) (USD $)
In Millions, unless otherwise specified
Mar. 31, 2012
Dec. 31, 2011
Regulatory Liabilities [Line Items]    
Non-Current Regulatory Liabilties $ 45.3 $ 43.5
Income Taxes [Member]
   
Regulatory Liabilities [Line Items]    
Non-Current Regulatory Liabilties 21.2 21.9
Plant Removal Obligations [Member]
   
Regulatory Liabilities [Line Items]    
Non-Current Regulatory Liabilties 15.9 15.0
Other [Member]
   
Regulatory Liabilities [Line Items]    
Non-Current Regulatory Liabilties $ 8.2 $ 6.6