Stockholders' Equity - Accumulated Other Comprehensive Loss (Details) - 2.5% (€650 million) senior notes due 2022 - USD ($) $ in Millions	9 Months Ended	12 Months Ended
	Sep. 30, 2019	Dec. 31, 2018
Accumulated Other Comprehensive Income (Loss) [Line Items]
Foreign currency translation gains (losses), net of tax	$ (2)	$ (27)
Foreign currency translation gains (losses), before tax	$ (2)	$ (35)
Debt, interest rate (percentage)	2.50%