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Debt - Unsecured Senior Notes, Interest Expense and Financing Costs, and Additional Information (Details) - USD ($) $ in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2019	Sep. 30, 2018	Sep. 30, 2019	Sep. 30, 2018	Dec. 31, 2018
Debt Instrument [Line Items]
Amount outstanding	$ 2,700		$ 2,700		$ 2,700
Interest expense	21	$ 32	64	$ 113
Amortization of financing costs and discounts	1	$ 1	3	$ 5
Total Unsecured Senior Notes
Debt Instrument [Line Items]
Interest payable	14		14		15
Unsecured Debt \| 2021 Notes
Debt Instrument [Line Items]
Amount outstanding	$ 650		$ 650		$ 650
Interest rate	2.30%		2.30%		2.30%
Unsecured Debt \| 2026 Notes
Debt Instrument [Line Items]
Amount outstanding	$ 850		$ 850		$ 850
Interest rate	3.40%		3.40%		3.40%
Unsecured Debt \| 2026 Notes \| Significant Other Observable Inputs (Level 2)
Debt Instrument [Line Items]
Fair value of notes					$ 800
Unsecured Debt \| 2022 Notes
Debt Instrument [Line Items]
Amount outstanding	$ 400		$ 400		$ 400
Interest rate	2.60%		2.60%		2.60%
Unsecured Debt \| 2027 Notes
Debt Instrument [Line Items]
Amount outstanding	$ 400		$ 400		$ 400
Interest rate	3.40%		3.40%		3.40%
Unsecured Debt \| 2027 Notes \| Significant Other Observable Inputs (Level 2)
Debt Instrument [Line Items]
Fair value of notes					$ 376
Unsecured Debt \| 2047 Notes
Debt Instrument [Line Items]
Amount outstanding	$ 400		$ 400		$ 400
Interest rate	4.50%		4.50%		4.50%
Unsecured Debt \| 2047 Notes \| Significant Other Observable Inputs (Level 2)
Debt Instrument [Line Items]
Fair value of notes	$ 454		$ 454		$ 360