Marketable Securities (Details) (USD $) In Millions, unless otherwise specified	Dec. 31, 2012	Dec. 31, 2011
Investments in marketable securities
Marketable securities, amortized cost	$ 131	$ 286
Marketable securities, Fair value	131	286
Corporate debt securities [Member]
Investments in marketable securities
Marketable securities, amortized cost	82	174
Marketable securities, Fair value	82	174
U.S. government and agency debt securities [Member]
Investments in marketable securities
Marketable securities, amortized cost	16	32
Marketable securities, Fair value	16	32
Foreign Government Debt Securities [Member]
Investments in marketable securities
Marketable securities, amortized cost	1	0
Marketable securities, Fair value	1	0
Asset-backed debt securities [Member]
Investments in marketable securities
Marketable securities, amortized cost	6	18
Marketable securities, Fair value	6	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	10	27
Marketable securities, Fair value	$ 10	$ 27