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Stockholders' Equity - Narrative (Details) - RSUs - Certain Employees - USD ($) shares in Thousands, $ in Millions	3 Months Ended		6 Months Ended
	Jun. 30, 2019	Jun. 30, 2018	Jun. 30, 2019	Jun. 30, 2018
Class of Stock [Line Items]
Shares granted (in shares)			2,449	1,071
Compensation expense	$ 5	$ 4	$ 9	$ 8
Tranche One
Class of Stock [Line Items]
Vesting percentage			33.30%
Tranche Two
Class of Stock [Line Items]
Vesting percentage			33.30%
Tranche Three
Class of Stock [Line Items]
Vesting percentage			33.40%