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SUMMARY OF SIGNIFICANT ACCOUNTING POLICIES - ASSET RETIREMENT OBLIGATIONS (Details) - USD ($)
$ in Millions
12 Months Ended
Dec. 31, 2020
Dec. 31, 2019
Asset retirement obligation    
Beginning balance $ 4,659 $ 1,499
Liabilities assumed in the Acquisition 0 3,370
Liabilities incurred – capitalized to PP&E 79 71
Liabilities settled and paid (186) (200)
Accretion expense 147 131
Acquisitions, divestitures and other, net (294) 0
WES loss of control 0 (359)
Revisions to previous estimates (275) 147
Ending balance 4,130 4,659
Asset retirement obligations, current balance included in Accrued Liabilities $ 153 $ 248