Stock-Based Compensation - Narrative (Details) - USD ($) $ in Millions	3 Months Ended	6 Months Ended
Jun. 30, 2019	Jun. 30, 2018	Jun. 30, 2019	Jun. 30, 2018
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Share based compensation expense	$ 3	$ 2	$ 6	$ 4
Employee Stock Purchase Plan \| 2017 ESPP
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Stock compensation shares authorized (in shares)	800,000		800,000
Shares available for issuance (in shares)	577,920		577,920
Shares issued during period (in shares)	26,785		59,663

Amount of expense for award under share-based payment arrangement. Excludes amount capitalized.

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