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Recurring Fair Value Measures, Activity in Level 3 (Details) (USD $)
In Millions, unless otherwise specified
12 Months Ended
Dec. 31, 2011
Dec. 31, 2010
Derivatives [Member]
   
Recurring Fair Value Measures, Activity in Level 3 [Line Items]    
Beginning Balance   $ 0.7
Settled During the Period   (0.7) [1]
Redeemed During the Period   0
Ending Balance   0
Debt Securities Issued by States of the United States (ARS) [Member]
   
Recurring Fair Value Measures, Activity in Level 3 [Line Items]    
Beginning Balance 6.7 6.7
Settled During the Period 0 0
Redeemed During the Period (6.7) [2] 0
Ending Balance $ 0 $ 6.7
[1] (a)During the second quarter of 2010, the $0.7 million of financial transmission rights derivatives were settled.
[2] (a)The ARS were redeemed at carrying value on January 5, 2011.