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Revenues and Accounts Receivable - Contract Balances (Details) - USD ($)
$ in Millions
3 Months Ended 6 Months Ended
Dec. 31, 2020
Jun. 30, 2021
Change In Contract With Customer, Liability [Roll Forward]    
Beginning balance   $ 501
Amounts recognized as revenue   (384)
Additions   22
Ending balance $ 501 $ 139
Crude Oil Sales Agreements Entered Into In Conjunction With Storage Arrangements And Future Inventory Exchanges    
Change In Contract With Customer, Liability [Roll Forward]    
Amounts recognized as revenue $ (361)