Marketable Securities (Details) (USD $) In Millions, unless otherwise specified	Mar. 31, 2012	Dec. 31, 2011
Investments in marketable securities
Marketable securities, amortized cost	$ 138	$ 286
Marketable securities, Fair value	138	286
Corporate debt securities [Member]
Investments in marketable securities
Marketable securities, amortized cost	86	174
Marketable securities, Fair value	86	174
U.S. government and agency debt securities [Member]
Investments in marketable securities
Marketable securities, amortized cost	12	32
Marketable securities, Fair value	12	32
Asset-backed debt securities [Member]
Investments in marketable securities
Marketable securities, amortized cost	9	18
Marketable securities, Fair value	9	18
Certificates of deposit [Member]
Investments in marketable securities
Marketable securities, amortized cost	16	35
Marketable securities, Fair value	16	35
Municipal debt securities [Member]
Investments in marketable securities
Marketable securities, amortized cost	15	27
Marketable securities, Fair value	$ 15	$ 27