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Regulatory Assets and Liabilities Regulatory Assets and Liabilities (Details) (USD $)
In Millions, unless otherwise specified
Dec. 31, 2011
Dec. 31, 2010
Regulatory Assets and Liabilities Disclosure [Abstract]    
Regulatory Assets Earning a Return, Other Category $ 21 $ 26
Regulatory Assets Earning a Return 22  
Regulatory Assets Earning a Rate of Return at the Cost of Capital 7  
Authorized Cost of Capital 8.033%  
Regulatory Assets Earning a Rate of Return at the Approved Rate 7  
Approved Rate for Deferred Account Under Amortization, Low End of Range 2.01%  
Approved Rate for Deferred Accounts under Amortization, High End of Range 4.27%  
Regulatory Assets Earning a Rate of Return by Inclusion in Rate Base 8  
Amount of Impairment to Carrying Amount of Regulatory Assets $ 3