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Long-Term Debt - Senior Notes (Details) - USD ($)
$ in Millions
12 Months Ended
Aug. 07, 2017
Dec. 31, 2017
Dec. 31, 2016
Dec. 31, 2015
Debt Instrument [Line Items]        
Non-cash accelerated discount and debt issuance cost   $ 38 $ 23 $ 16
Outstanding senior notes   2,249 2,703  
Senior Notes        
Debt Instrument [Line Items]        
Outstanding senior notes   0 $ 1,050  
Senior Notes | 7.25% Senior Notes, Maturing October 2022        
Debt Instrument [Line Items]        
Senior notes redeemed   1,100    
Interest rate on senior notes 7.25%      
Early termination make whole premium   $ 65    
Non-cash accelerated discount and debt issuance cost $ 16