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Basis of Presentation - Narrative (Details)
$ / shares in Units, shares in Millions, $ in Millions
3 Months Ended 9 Months Ended
Sep. 30, 2018
USD ($)
$ / shares
shares
Sep. 30, 2017
USD ($)
Sep. 30, 2018
USD ($)
$ / shares
$ / MMBTU
shares
Sep. 30, 2017
USD ($)
Mar. 31, 2018
USD ($)
Feb. 15, 2018
USD ($)
Dec. 31, 2017
USD ($)
Apr. 24, 2017
USD ($)
Finite-Lived Intangible Assets [Line Items]                
Goodwill $ 1,401   $ 1,401       $ 1,310 $ 1,291
Intangible assets, gross 318   318          
Amortization expense $ 8   $ 22          
Share purchase plan, authorized amount           $ 750    
Shares repurchased and retired during period (in shares) | shares 3.4   7.4          
Repurchase price (in usd per share) | $ / shares $ 30.07   $ 31.34          
Revenues $ 1,273 $ 960 $ 3,789 $ 3,055        
Expenses 775 $ 958 2,240 $ 5,318        
Stranded tax assets 7   $ 7          
Minimum                
Finite-Lived Intangible Assets [Line Items]                
Intangible asset, useful life     7 years          
Maximum                
Finite-Lived Intangible Assets [Line Items]                
Intangible asset, useful life     13 years          
ASC 606                
Finite-Lived Intangible Assets [Line Items]                
Revenues (2)   $ 5          
Expenses (2)   $ 5          
West Africa                
Finite-Lived Intangible Assets [Line Items]                
Contractual sales price (in usd per MMBtu) | $ / MMBTU     0.25          
Retained Earnings | ASC 606                
Finite-Lived Intangible Assets [Line Items]                
Cumulative increase (decrease) effect of adoption $ (2)   $ (2)   $ 5