Stockholders' Equity - Accumulated Other Comprehensive Loss (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017	Sep. 30, 2016	Jun. 30, 2015
Equity [Abstract]
Foreign currency translation adjustments, net of tax	$ (220)	$ (142)
Net unrealized gain (loss) on available for sale securities, net of tax	0	(7)
Accumulated other comprehensive loss	(220)	(149)
Accumulated Other Comprehensive Income (Loss) [Line Items]
Net unrealized loss on available-for-sale-securities before tax		(9)
2.5% Senior Notes Due 2022
Accumulated Other Comprehensive Income (Loss) [Line Items]
Foreign currency translation gains (losses), net of tax	(27)	(45)
Foreign currency translation gains (losses), before tax	$ (35)	$ (71)
Senior notes, interest rate	2.50%	2.50%	2.50%	2.50%