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Fair Value Measurements (Level 3 Reconciliation) (Details) (Fair Value Measurements, Level 3 [Member], Price Risk Management Instruments [Member], USD $)
In Millions, unless otherwise specified
3 Months Ended
Mar. 31, 2012
Mar. 31, 2011
Fair Value Measurements, Level 3 [Member] | Price Risk Management Instruments [Member]
   
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]    
Liability balance as of January 1 $ (74) $ (399)
Realized and unrealized gains (losses): Included in regulatory assets and liabilities or balancing accounts (25) [1] 87 [1]
Liability balance as of March 31 $ (99) $ (312)
[1] Includes the effect of the contractual ability to settle contracts under master netting agreements and margin cash collateral.