Available-For-Sale Investments (Narrative) (Details) (USD $)	12 Months Ended
	Dec. 31, 2011	Dec. 31, 2010	Dec. 31, 2009
Schedule Of Available-For-Sale Securities [Line Items]
Impairment loss for securities that have credit losses	$ 18,000,000	$ 16,000,000
Proceeds from sales and maturities of short term investments	3,200,000,000	2,100,000,000	1,300,000,000
Asset-Backed Securities [Member]
Schedule Of Available-For-Sale Securities [Line Items]
Estimation of credit losses	$ 1,000,000	$ 2,000,000	$ 2,000,000