Stockholders' Equity - Accumulated Other Comprehensive Loss (Details) - USD ($) $ in Millions	6 Months Ended	12 Months Ended
	Jun. 30, 2018	Dec. 31, 2017
Accumulated Other Comprehensive Income (Loss) [Line Items]
Foreign currency translation adjustments, net of tax	$ (190)	$ (142)
Net unrealized loss on available for sale securities, net of tax(2)	0	(7)
Accumulated other comprehensive loss	(190)	(149)
2.5% Senior Notes Due 2022
Accumulated Other Comprehensive Income (Loss) [Line Items]
Foreign currency translation gains (losses), net of tax	(39)	(45)
Foreign currency translation gains (losses), before tax	$ (51)	(71)
Debt, interest rate (percentage)	2.50%
Available-for-sale Securities
Accumulated Other Comprehensive Income (Loss) [Line Items]
Gross unrealized loss		$ 9