Stockholders' Equity - Narrative (Details) - RSUs - Certain Employees - USD ($) shares in Thousands, $ in Millions	3 Months Ended
Mar. 31, 2019	Mar. 31, 2018
Class of Stock [Line Items]
Shares granted (in shares)	1,873	934
Compensation expense	$ 4	$ 4
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%

Represents the expense recognized during the period arising from equity-based compensation arrangements (for example, shares of stock, unit, stock options or other equity instruments) with employees, directors and certain consultants qualifying for treatment as employees.

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Line items represent financial concepts included in a table. These concepts are used to disclose reportable information associated with domain members defined in one or many axes to the table.

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The number of grants made during the period on other than stock (or unit) option plans (for example, phantom stock or unit plan, stock or unit appreciation rights plan, performance target plan).

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Percentage of vesting of share-based compensation awards.

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