Debt - Summary of Debt (Details) - USD ($) $ in Millions	Mar. 31, 2019	Dec. 31, 2018
Debt Instrument [Line Items]
Gross Carrying Amount	$ 2,700	$ 2,700
Unamortized Discount and Deferred Financing Costs	(28)	(29)
Net Carrying Amount	2,672	2,671
Unsecured Debt | 2021 Notes
Debt Instrument [Line Items]
Gross Carrying Amount	650	650
Unamortized Discount and Deferred Financing Costs	(3)	(3)
Net Carrying Amount	647	647
Unsecured Debt | 2022 Notes
Debt Instrument [Line Items]
Gross Carrying Amount	400	400
Unamortized Discount and Deferred Financing Costs	(3)	(3)
Net Carrying Amount	397	397
Unsecured Debt | 2026 Notes
Debt Instrument [Line Items]
Gross Carrying Amount	850	850
Unamortized Discount and Deferred Financing Costs	(8)	(8)
Net Carrying Amount	842	842
Unsecured Debt | 2027 Notes
Debt Instrument [Line Items]
Gross Carrying Amount	400	400
Unamortized Discount and Deferred Financing Costs	(4)	(5)
Net Carrying Amount	396	395
Unsecured Debt | 2047 Notes
Debt Instrument [Line Items]
Gross Carrying Amount	400	400
Unamortized Discount and Deferred Financing Costs	(10)	(10)
Net Carrying Amount	$ 390	$ 390