Fair Value of Financial Instruments Unobservable Input Reconciliation (Details) (USD $) In Millions, unless otherwise specified	3 Months Ended		9 Months Ended
	Sep. 30, 2012	Sep. 30, 2011	Sep. 30, 2012	Sep. 30, 2011
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Net liabilities from price risk management activities as of beginning of period	$ 88	$ 127	$ 79	$ 120
Realized and unrealized losses, net	(7)	21	4	29
Purchases	(2)	1	(2)	1
Issues	0	0	(1)	0
Settlements	0	0	0	(1)
Transfers out of Level 3 to Level 2	0	0	(1)	0
Net liabilities from price risk management activities as of end of period	$ 79	$ 149	$ 79	$ 149