Fair Value of Financial Instruments Unobservable Input Reconciliation (Details) (USD $) In Millions, unless otherwise specified	3 Months Ended		6 Months Ended
	Jun. 30, 2013	Jun. 30, 2012	Jun. 30, 2013	Jun. 30, 2012
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Balance as of the beginning of the period	$ 45	$ 95	$ 16	$ 79
Net realized and unrealized losses (1)	(11)	(7)	(15)	(11)
Purchases	0	0	25	0
Issuances	0	0	0	(1)
Transfers out of Level 3 to Level 2	0	0	0	(1)
Balance as of the end of the period	$ 56	$ 88	$ 56	$ 88