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Basis of Presentation and Summary of Significant Accounting Policies - Capitalized Costs of Unproved Properties (Details) - USD ($)
$ in Millions
12 Months Ended
Dec. 31, 2018
Dec. 31, 2017
Dec. 31, 2016
Dec. 31, 2015
Accounting Policies [Abstract]        
Leasehold cost $ 24 $ 31 $ 40 $ 1,577
Leasehold cost, total 1,672      
Exploration cost 122 0 2 0
Exploration cost, total 124      
Capitalized interest 125 84 63 269
Capitalized interest, total 541      
Unproved properties 271 115 $ 105 $ 1,846
Unproved properties, total $ 2,337 $ 3,484