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Variable Interest Entities (Detail) - Variable Interest Entity, Primary Beneficiary [Member] - Western Gas Partners, LP [Member] - USD ($)
$ in Millions
1 Months Ended 3 Months Ended
May 31, 2017
May 31, 2008
Mar. 31, 2018
Mar. 31, 2017
Variable Interest Entity [Line Items]        
Note receivable for WES   $ 260    
Debt instrument, term   30 years    
Fixed annual rate   6.50%    
Interest income for WES     $ 4 $ 4
Proceeds from settlement of related party obligation $ 37      
Above-market component of swap extensions with Anadarko     $ 14 $ 12