Debt - Summary of Debt (Details) - USD ($) $ in Millions	Dec. 31, 2018	Dec. 31, 2017
Debt Instrument [Line Items]
Gross Carrying Amount	$ 2,700	$ 4,440
Unamortized Discount and Deferred Financing Costs	(29)	(50)
Net Carrying Amount	2,671	4,390
Credit Facilities | 2017 TLA
Debt Instrument [Line Items]
Gross Carrying Amount		990
Unamortized Discount and Deferred Financing Costs		(8)
Net Carrying Amount		982
Unsecured Notes | 2021 Notes
Debt Instrument [Line Items]
Gross Carrying Amount	650	650
Unamortized Discount and Deferred Financing Costs	(3)	(4)
Net Carrying Amount	647	646
Unsecured Notes | 2022 Notes
Debt Instrument [Line Items]
Gross Carrying Amount	400	400
Unamortized Discount and Deferred Financing Costs	(3)	(4)
Net Carrying Amount	397	396
Unsecured Notes | 2023 Notes
Debt Instrument [Line Items]
Gross Carrying Amount		750
Unamortized Discount and Deferred Financing Costs		(9)
Net Carrying Amount		741
Unsecured Notes | 2026 Notes
Debt Instrument [Line Items]
Gross Carrying Amount	850	850
Unamortized Discount and Deferred Financing Costs	(8)	(9)
Net Carrying Amount	842	841
Unsecured Notes | 2027 Notes
Debt Instrument [Line Items]
Gross Carrying Amount	400	400
Unamortized Discount and Deferred Financing Costs	(5)	(6)
Net Carrying Amount	395	394
Unsecured Notes | 2047 Notes
Debt Instrument [Line Items]
Gross Carrying Amount	400	400
Unamortized Discount and Deferred Financing Costs	(10)	(10)
Net Carrying Amount	$ 390	$ 390