Marketable Securities (Details) (USD $) In Millions, unless otherwise specified	Jun. 30, 2014	Dec. 31, 2013
Investments in marketable securities
Marketable securities, amortized cost	$ 138	$ 142
Marketable securities, fair value	138	142
Corporate debt securities [Member]
Investments in marketable securities
Marketable securities, amortized cost	92	87
Marketable securities, fair value	92	87
U.S. government and agency debt securities [Member]
Investments in marketable securities
Marketable securities, amortized cost	4	12
Marketable securities, fair value	4	12
Asset-backed debt securities [Member]
Investments in marketable securities
Marketable securities, amortized cost	17	20
Marketable securities, fair value	17	20
Certificates of deposit [Member]
Investments in marketable securities
Marketable securities, amortized cost	19	17
Marketable securities, fair value	19	17
Municipal debt securities [Member]
Investments in marketable securities
Marketable securities, amortized cost	6	6
Marketable securities, fair value	$ 6	$ 6