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Regulatory Matters - The Patient Protection and Affordable Care Act of 2010 (PPACA) (Details) (USD $)
In Millions, unless otherwise specified
 3 Months Ended 12 Months Ended
Jun. 30, 2011
Dec. 31, 2013
Dec. 31, 2012
Dec. 31, 2011
Dec. 31, 2010
Unusual or Infrequent Item [Line Items]          
Deferred Income Tax Benefit   $ 28.6 $ 37.5 $ 35.8  
Non-Current Regulatory Asset   263.8 340.3    
PPACA Elimination of Medicare Part D [Member]
          
Unusual or Infrequent Item [Line Items]          
Reversal of Previously Recorded Tax Benefit         4.0
Deferred Income Tax Benefit 2.9 0 0 (2.9)  
Non-Current Regulatory Asset $ 5.0