Marketable Securities (Details) - USD ($) $ in Millions	Mar. 31, 2016	Dec. 31, 2015
Investments in marketable securities
Amortized Cost	$ 226	$ 230
Fair Value	226	230
Corporate debt securities
Investments in marketable securities
Amortized Cost	134	134
Fair Value	134	134
U.S. government and agency debt securities
Investments in marketable securities
Amortized Cost	52	57
Fair Value	52	57
Asset-backed debt securities
Investments in marketable securities
Amortized Cost	13	21
Fair Value	13	21
Certificates of deposit
Investments in marketable securities
Amortized Cost	26	15
Fair Value	26	15
Municipal debt securities
Investments in marketable securities
Amortized Cost	1	3
Fair Value	$ 1	$ 3