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Fair Value (Reconciliation of Level 3 Assets and Liabilities at Fair Value on a Recurring Basis) (Details) (USD $)
In Millions, unless otherwise specified
3 Months Ended
Mar. 31, 2014
Mar. 31, 2013
Fair value reconciliation of Level 3 assets and liabilities measured at fair value on a recurring basis    
Beginning Balance $ 3 $ 1
Recorded in regulatory assets/liabilities 4 1
Purchases issuances and settlements [Abstract]    
Settlements (6) (1)
Ending Balance 1 1
Fair Value, Measurement with Unobservable Inputs Reconciliation, Recurring Basis, Asset, Period Increase (Decrease) $ 0 $ 1