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Debt - Unsecured Senior Notes and Interest Expense and Financing Costs (Details) - USD ($)
$ in Millions
12 Months Ended
Dec. 31, 2019
Dec. 31, 2018
Dec. 31, 2017
Debt Instrument [Line Items]      
Amount outstanding $ 2,700 $ 2,700  
Interest expense 86 134 $ 150
Amortization of debt discount and deferred financing costs 4 6 $ 12
Notes      
Debt Instrument [Line Items]      
Accrued interest payable $ 15 15  
Unsecured Notes      
Debt Instrument [Line Items]      
Redemption percentage of aggregate principal amount plus a make-whole premium 100.00%    
Redemption price percentage 100.00%    
Percentage of principal repayable to option holders upon certain criteria 101.00%    
Unsecured Notes | 2021 Notes      
Debt Instrument [Line Items]      
Amount outstanding $ 650 $ 650  
Interest rate 2.30% 2.30%  
Unsecured Notes | 2022 Notes      
Debt Instrument [Line Items]      
Amount outstanding $ 400 $ 400  
Interest rate 2.60% 2.60%  
Unsecured Notes | 2026 Notes      
Debt Instrument [Line Items]      
Amount outstanding $ 850 $ 850  
Interest rate 3.40% 3.40%  
Unsecured Notes | 2027 Notes      
Debt Instrument [Line Items]      
Amount outstanding $ 400 $ 400  
Interest rate 3.40% 3.40%  
Unsecured Notes | 2047 Notes      
Debt Instrument [Line Items]      
Amount outstanding $ 400 $ 400  
Interest rate 4.50% 4.50%