Marketable Securities (Details) (USD $) In Millions, unless otherwise specified	Mar. 31, 2013	Dec. 31, 2012
Investments in marketable securities
Marketable securities, amortized cost	$ 131	$ 131
Marketable securities, Fair value	131	131
Corporate debt securities [Member]
Investments in marketable securities
Marketable securities, amortized cost	80	82
Marketable securities, Fair value	80	82
U.S. government and agency debt securities [Member]
Investments in marketable securities
Marketable securities, amortized cost	21	16
Marketable securities, Fair value	21	16
Foreign Government Debt Securities [Member]
Investments in marketable securities
Marketable securities, amortized cost	1	1
Marketable securities, Fair value	1	1
Asset-backed debt securities [Member]
Investments in marketable securities
Marketable securities, amortized cost	6	6
Marketable securities, Fair value	6	6
Certificates of deposit [Member]
Investments in marketable securities
Marketable securities, amortized cost	14	16
Marketable securities, Fair value	14	16
Municipal debt securities [Member]
Investments in marketable securities
Marketable securities, amortized cost	9	10
Marketable securities, Fair value	$ 9	$ 10