Marketable Securities (Details) - USD ($) $ in Millions	Jun. 30, 2015	Dec. 31, 2014
Investments in marketable securities
Amortized Cost	$ 82	$ 154
Fair Value	82	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	2	22
Fair Value	2	22
Asset-backed debt securities
Investments in marketable securities
Amortized Cost	12	17
Fair Value	12	17
Certificates of deposit
Investments in marketable securities
Amortized Cost	9	18
Fair Value	9	18
Municipal debt securities
Investments in marketable securities
Amortized Cost	2	4
Fair Value	$ 2	$ 4