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Other Changes in Pension and Postretirement Health and Life Plan Assets and Benefit Obligations Recognized in Other Comprehensive Income and Regulatory Assets (Details) (USD $)
In Millions, unless otherwise specified
 12 Months Ended
Dec. 31, 2011
Dec. 31, 2010
Pension [Member]
    
Defined Benefit Plans and Other Postretirement Benefit Plans [Line Items]    
Net Loss $ 56.1 $ 35.2
Amortization of Prior Service Cost (0.3) (0.5)
Amortization of Gain (Loss) (12.2) (6.6)
Total Recognized in Other Comprehensive Income and Regulatory Assets 43.6 28.1
Postretirement Health and Life [Member]
    
Defined Benefit Plans and Other Postretirement Benefit Plans [Line Items]    
Net Loss 6.9 15.3
Prior Service Cost (Credit) Arising During Period 0 (14.2)
Amortization of Prior Service Cost 1.7 0.1
Amortization of Transition Obligation (0.1) (2.5)
Amortization of Gain (Loss) (8.5) (4.8)
Total Recognized in Other Comprehensive Income and Regulatory Assets $ 0 $ (6.1)