Marketable Securities - Investments in Marketable Securities (Details) - USD ($) $ in Millions	Dec. 31, 2015	Dec. 31, 2014
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	$ 230	$ 154
Fair Value	230	154
Corporate debt securities
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	134	93
Fair Value	134	93
U.S. government and agency debt securities
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	57	22
Fair Value	57	22
Asset-backed debt securities
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	21	17
Fair Value	21	17
Certificates of deposit
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	15	18
Fair Value	15	18
Municipal debt securities
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	3	4
Fair Value	$ 3	$ 4