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Asset Retirement Obligations (Details) - USD ($)
$ in Millions
3 Months Ended 6 Months Ended
Jun. 30, 2019
Jun. 30, 2018
Jun. 30, 2019
Jun. 30, 2018
Dec. 31, 2018
Changes in ARO liability          
Asset retirement obligations, beginning of period     $ 136 $ 21  
Additional liabilities incurred     2 2  
Liabilities acquired     3 0  
Liabilities settled     (4) (1)  
Asset retirement obligation accretion $ 3 $ 0 5 1  
Asset retirement obligations, end of period 142 23 142 23  
Less current portion 0 1 0 1  
Asset retirement obligations - long-term $ 142 $ 22 $ 142 $ 22 $ 136