Risk Management and Fair Values (Reconciliation Of Changes In The Net Assets (Liabilities) For The Fair Value Of Derivatives Classified As Level 3 In The Fair Value Hierarchy) (Details) (USD $) In Millions, unless otherwise specified	12 Months Ended
	Dec. 31, 2012	Dec. 31, 2011	Dec. 31, 2010
Reconciliation of changes in the net assets (liabilities) for the fair value of derivatives classified as Level 3 in the fair value hierarchy
Balance as of Beginning of Period	$ 312	$ 197	$ 200
Unrealized gains from price changes	139	274	220
Unrealized gains on originations	9	15	(4)
Realized gains (losses) included in earning	(14)	(6)	1
Realized gains on settlements	(268)	(168)	(220)
Balance as of End of Period	$ 178	$ 312	$ 197