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Fair Value of FInancial Instruments Fair Value Unobservable Input Reconciliation (Details) (USD $)
In Millions, unless otherwise specified
 12 Months Ended
Dec. 31, 2012
Dec. 31, 2011
Dec. 31, 2010
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]      
Net liabilities from price risk management activities as of beginning of year $ 79 $ 120 $ 154
Net realized and unrealized losses 15 86 65
Purchases, issuances, and settlements, net     27
Purchases (1) (3)  
Fair Value, Measurement with Unobservable Inputs Reconciliation, Recurring Basis, Liability, Issues (1) 0  
Settlements 0 (1)  
Net transfers out of Level 3 to Level 2 (76) (129) (126)
Net liabilities from price risk management activities as of end of year 16 79 120
Level 3 net unrealized losses that have been fully offset by the effect of regulatory accounting 14 88 95
Net realized losses $ 1