Debt - Summary of Debt (Details) - USD ($) $ in Millions	Mar. 31, 2017	Dec. 31, 2016
Debt Instrument [Line Items]
Gross Carrying Amount	$ 4,440	$ 4,940
Unamortized Discount and Deferred Financing Costs	(47)	(53)
Net Carrying Amount	4,393	4,887
Line of Credit | 2017 TLA
Debt Instrument [Line Items]
Gross Carrying Amount	2,190
Unamortized Discount and Deferred Financing Costs	(22)
Net Carrying Amount	2,168
Line of Credit | 2016 TLA
Debt Instrument [Line Items]
Gross Carrying Amount		2,690
Unamortized Discount and Deferred Financing Costs		(27)
Net Carrying Amount		2,663
Unsecured Debt | 2021 Notes
Debt Instrument [Line Items]
Gross Carrying Amount	650	650
Unamortized Discount and Deferred Financing Costs	(5)	(5)
Net Carrying Amount	645	645
Unsecured Debt | 2023 Notes
Debt Instrument [Line Items]
Gross Carrying Amount	750	750
Unamortized Discount and Deferred Financing Costs	(10)	(11)
Net Carrying Amount	740	739
Unsecured Debt | 2026 Notes
Debt Instrument [Line Items]
Gross Carrying Amount	850	850
Unamortized Discount and Deferred Financing Costs	(10)	(10)
Net Carrying Amount	$ 840	$ 840