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Asset Retirement Obligation (Details) - USD ($)
$ in Millions
12 Months Ended
Dec. 31, 2019
Dec. 31, 2018
Asset retirement obligation    
Balance at beginning of year $ 307 $ 283
Additional liabilities accrued 13 2
Revisions in estimated cash outflows 2 6
Acquisition of assets 1 0
Disposition of assets (1) (1)
Accretion expense 20 18
Transferred to Liabilities held for sale 0 (1)
Balance at end of year $ 342 $ 307