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Regulatory Matters - Regulatory Assets (Details) (USD $)
In Millions
Sep. 30, 2011
Dec. 31, 2010
Regulatory Assets [Line Items]  
Current Regulatory Assets$ 17.1[1]$ 20.6[1]
Non-Current Regulatory Assets286.9310.2
Regulatory Assets304.0330.8
Deferred Fuel [Member]
  
Regulatory Assets [Line Items]  
Current Regulatory Assets17.120.6
Future Benefit Obligations Under Defined Benefit Pension and Other Postretirement Benefit Plans [Member]
  
Regulatory Assets [Line Items]  
Non-Current Regulatory Assets245.3257.9
Boswell Unit 3 Environmental Rider [Member]
  
Regulatory Assets [Line Items]  
Non-Current Regulatory Assets020.5
Income Taxes [Member]
  
Regulatory Assets [Line Items]  
Non-Current Regulatory Assets24.517.3
Asset Retirement Obligation [Member]
  
Regulatory Assets [Line Items]  
Non-Current Regulatory Assets9.27.8
Medicare Part D [Member]
  
Regulatory Assets [Line Items]  
Non-Current Regulatory Assets5.00
Other [Member]
  
Regulatory Assets [Line Items]  
Non-Current Regulatory Assets$ 2.9$ 6.7
[1]Current regulatory assets are included in prepayments and other on the consolidated balance sheet.