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Regulatory Matters - Regulatory Assets and Liabilities (Details) (USD $)
In Millions, unless otherwise specified
Jun. 30, 2012
Dec. 31, 2011
Jun. 30, 2012
Income Taxes [Member]
Dec. 31, 2011
Income Taxes [Member]
Jun. 30, 2012
Plant Removal Obligations [Member]
Dec. 31, 2011
Plant Removal Obligations [Member]
Jun. 30, 2012
Wholesale and Retail Contra AFUDC [Member]
Dec. 31, 2011
Wholesale and Retail Contra AFUDC [Member]
Jun. 30, 2012
Other [Member]
Dec. 31, 2011
Other [Member]
Jun. 30, 2012
Deferred Fuel [Member]
Dec. 31, 2011
Deferred Fuel [Member]
Jun. 30, 2012
Future Benefit Obligations Under Defined Benefit Pension and Other Postretirement Benefit Plans [Member]
Dec. 31, 2011
Future Benefit Obligations Under Defined Benefit Pension and Other Postretirement Benefit Plans [Member]
Jun. 30, 2012
Income Taxes [Member]
Dec. 31, 2011
Income Taxes [Member]
Jun. 30, 2012
Asset Retirement Obligation [Member]
Dec. 31, 2011
Asset Retirement Obligation [Member]
Jun. 30, 2012
Current Cost Recovery Riders [Member]
Jun. 30, 2011
Current Cost Recovery Riders [Member]
Jun. 30, 2012
PPACA Income Tax Deferral [Member]
Dec. 31, 2011
PPACA Income Tax Deferral [Member]
Jun. 30, 2012
Conservation Improvement Program [Member]
Dec. 31, 2011
Conservation Improvement Program [Member]
Jun. 30, 2012
Other [Member]
Dec. 31, 2011
Other [Member]
Regulatory Assets and Liabilities [Line Items]                                                    
Current Regulatory Assets $ 18.5 $ 17.5                 $ 18.5 [1] $ 17.5 [1]                            
Non-Current Regulatory Assets 336.9 345.9                     282.0 292.8 28.3 28.6 10.9 9.8 5.9 0.7 5.0 5.0 0.5 4.6 4.3 4.4
Total Regulatory Assets 355.4 363.4                                                
Non-Current Regulatory Liabilities $ 49.3 $ 43.5 $ 20.5 $ 21.9 $ 16.7 $ 15.0 $ 6.3 $ 1.5 $ 5.8 $ 5.1                                
[1] Current regulatory assets are included in prepayments and other on the Consolidated Balance Sheet.