Marketable Securities (Details) (USD $) In Millions, unless otherwise specified	Dec. 31, 2014	Dec. 31, 2013
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	$ 154	$ 142
Fair Value	154	142
Corporate debt securities
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	93	87
Fair Value	93	87
U.S. government and agency debt securities
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	22	12
Fair Value	22	12
Asset-backed debt securities
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	17	20
Fair Value	17	20
Certificates of deposit
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	18	17
Fair Value	18	17
Municipal debt securities
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	4	6
Fair Value	$ 4	$ 6