Marketable Securities - Investments in Marketable Securities (Details) - USD ($) $ in Millions	Dec. 31, 2017	Dec. 31, 2016
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	$ 99	$ 91
Fair Value	99	91
Corporate debt securities
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	68	69
Fair Value	68	69
U.S. government and agency debt securities
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	6	14
Fair Value	6	14
Asset-backed debt securities
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	11	2
Fair Value	11	2
Certificates of deposit
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	13	6
Fair Value	13	6
Municipal debt securities
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	1	0
Fair Value	$ 1	$ 0