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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, 2011
Dec. 31, 2010
Dec. 31, 2009
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]      
Net liabilities from price risk management activities as of beginning of year $ 120 $ 154 $ 123
Net realized and unrealized losses 86 65 47
Purchases, issuances, and settlements, net   27 0
Purchases 3    
Settlements (1)    
Net transfers out of Level 3 to Level 2 (129) (126) (16)
Net liabilities from price risk management activities as of end of year 79 120 154
Level 3 net unrealized losses that have been fully offset by the effect of regulatory accounting $ 88 $ 95 $ 49