Marketable Securities (Details) (USD $) In Millions, unless otherwise specified	Mar. 31, 2015	Dec. 31, 2014
Investments in marketable securities
Amortized Cost	$ 89	$ 154
Fair Value	89	154
Corporate debt securities
Investments in marketable securities
Amortized Cost	57	93
Fair Value	57	93
U.S. government and agency debt securities
Investments in marketable securities
Amortized Cost	6	22
Fair Value	6	22
Asset-backed debt securities
Investments in marketable securities
Amortized Cost	13	17
Fair Value	13	17
Certificates of deposit
Investments in marketable securities
Amortized Cost	11	18
Fair Value	11	18
Municipal debt securities
Investments in marketable securities
Amortized Cost	2	4
Fair Value	$ 2	$ 4