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Debt - Unsecured Senior Notes, Interest Expense and Financing Costs, and Additional Information (Details) - USD ($)
$ in Millions
3 Months Ended
Mar. 31, 2019
Mar. 31, 2018
Dec. 31, 2018
Debt Instrument [Line Items]      
Amount outstanding $ 2,700   $ 2,700
Interest expense 21 $ 40  
Amortization of financing costs and discounts 1 $ 2  
Total Unsecured Senior Notes      
Debt Instrument [Line Items]      
Interest payable 14   15
Unsecured Debt | 2021 Notes      
Debt Instrument [Line Items]      
Amount outstanding $ 650   $ 650
Interest rate 2.30%   2.30%
Unsecured Debt | 2026 Notes      
Debt Instrument [Line Items]      
Amount outstanding $ 850   $ 850
Interest rate 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
Interest rate 2.60%   2.60%
Unsecured Debt | 2027 Notes      
Debt Instrument [Line Items]      
Amount outstanding $ 400   $ 400
Interest rate 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
Interest rate 4.50%   4.50%
Unsecured Debt | 2047 Notes | Significant Other Observable Inputs (Level 2)      
Debt Instrument [Line Items]      
Fair value of notes $ 378   $ 360