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Debt - Unsecured Senior Notes and Interest Expense and Financing Costs (Details) - USD ($)
$ in Millions
12 Months Ended
Dec. 31, 2017
Dec. 31, 2016
Dec. 31, 2015
May 26, 2017
Debt Instrument [Line Items]        
Amount outstanding $ 4,440 $ 4,940    
Debt discount and issuance costs 50 53    
Interest expense 150 197 $ 193  
Amortization of debt discount and deferred financing costs $ 12 20 $ 7  
Unsecured Notes        
Debt Instrument [Line Items]        
Percentage of principal repayable to option holders upon certain criteria 101.00%      
Interest Payable $ 28 25    
Unsecured Notes | 2023 Notes        
Debt Instrument [Line Items]        
Amount outstanding $ 750 $ 750    
Interest rate   6.125%    
Redemption percentage of aggregate principal amount plus a make-whole premium and accrued and unpaid interest 100.00%      
Maximum percentage of outstanding Notes that can be redeemed with net cash proceeds from one or more qualified equity offerings 35.00%      
Debt discount and issuance costs $ 9 $ 11    
Unsecured Notes | 2021 Notes        
Debt Instrument [Line Items]        
Amount outstanding $ 650 $ 650    
Interest rate   2.30%    
Redemption percentage of aggregate principal amount plus a make-whole premium and accrued and unpaid interest 100.00%      
Redemption percentage of aggregate principal amount plus accrued and unpaid interest 100.00%      
Debt discount and issuance costs $ 4 $ 5    
Unsecured Notes | 2022 Notes        
Debt Instrument [Line Items]        
Amount outstanding $ 400     $ 400
Interest rate       2.60%
Redemption percentage of aggregate principal amount plus a make-whole premium and accrued and unpaid interest 100.00%      
Redemption percentage of aggregate principal amount plus accrued and unpaid interest 100.00%      
Debt discount and issuance costs $ 4      
Unsecured Notes | 2026 Notes        
Debt Instrument [Line Items]        
Amount outstanding $ 850 $ 850    
Interest rate   3.40%    
Redemption percentage of aggregate principal amount plus a make-whole premium and accrued and unpaid interest 100.00%      
Redemption percentage of aggregate principal amount plus accrued and unpaid interest 100.00%      
Debt discount and issuance costs $ 9 $ 10    
Unsecured Notes | 2027 Notes        
Debt Instrument [Line Items]        
Amount outstanding $ 400     $ 400
Interest rate       3.40%
Redemption percentage of aggregate principal amount plus a make-whole premium and accrued and unpaid interest 100.00%      
Redemption percentage of aggregate principal amount plus accrued and unpaid interest 100.00%      
Debt discount and issuance costs $ 6      
Unsecured Notes | 2047 Notes        
Debt Instrument [Line Items]        
Amount outstanding $ 400     $ 400
Interest rate       4.50%
Redemption percentage of aggregate principal amount plus a make-whole premium and accrued and unpaid interest 100.00%      
Redemption percentage of aggregate principal amount plus accrued and unpaid interest 100.00%      
Debt discount and issuance costs $ 10      
Credit Facilities | 2022, 2027, and 2047 Notes | Long-term debt, net        
Debt Instrument [Line Items]        
Debt discount and issuance costs       $ 20