Marketable Securities (Contractual maturity) (Details) (USD $) In Millions, unless otherwise specified	Sep. 30, 2013	Dec. 31, 2012
Marketable securities by contractual maturity
Due in one year or less	$ 436
Due after one year through five years	2,032
Due after five years through ten years	50
Due after ten years	0
Total marketable securities	$ 2,518	$ 2,810