Marketable Securities (Details) - USD ($) $ in Millions	Sep. 30, 2015	Dec. 31, 2014
Investments in marketable securities
Amortized Cost	$ 117	$ 154
Fair Value	117	154
Corporate debt securities
Investments in marketable securities
Amortized Cost	79	93
Fair Value	79	93
U.S. government and agency debt securities
Investments in marketable securities
Amortized Cost	9	22
Fair Value	9	22
Asset-backed debt securities
Investments in marketable securities
Amortized Cost	15	17
Fair Value	15	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	3	4
Fair Value	$ 3	$ 4