Cash, Cash Equivalents and Investments - Securities Available-for-Sale (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	$ 38	$ 52
Gross Unrealized Gains	0	0
Gross Unrealized Losses	0	0
Fair Value	38	52
U.S. treasuries
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	37	51
Gross Unrealized Gains	0	0
Gross Unrealized Losses	0	0
Fair Value	37	51
Exchange traded fund
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	1	1
Gross Unrealized Gains	0	0
Gross Unrealized Losses	0	0
Fair Value	$ 1	$ 1
Minimum | U.S. treasuries
Schedule of Available-for-sale Securities [Line Items]
Maturity	1 year	1 year
Maximum | U.S. treasuries
Schedule of Available-for-sale Securities [Line Items]
Maturity	5 years	5 years