Fair Value Measurements (Narrative) (Details) (USD $)	9 Months Ended
	Sep. 30, 2011	Sep. 30, 2010
Fair Value Measurements [Abstract]
Net assets measured at fair value on a recurring basis	$ 1,500,000,000	$ 1,700,000,000
Net assets measured at fair value on a recurring basis measured using unobservable inputs	4,000,000	197,000,000
Level 3 net assets as a percentage of total assets	1.00%
Assets transferred from Level 3 to Level 2	$ 8,000,000