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ASSET RETIREMENT OBLIGATIONS - Asset Retirement Obligation Roll-forward (Details) - USD ($)
$ in Millions
12 Months Ended
Dec. 31, 2020
Dec. 31, 2019
Asset Retirement Obligation [Roll Forward]    
Asset Retirement Obligation at Beginning of Year $ 165 $ 172
Asset Retirement Obligation, Liabilities Assumed, Acquisition 172 0
Asset Retirement Obligation, Accretion Expense 14 10
Asset Retirement Obligation, Liabilities Settled (9) (1)
Asset Retirement Obligation, Revision of Estimate   (16)
Asset Retirement Obligation at End of Year $ 342 $ 165