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Asset Retirement Obligations (Tables)
 6 Months Ended
Jun. 30, 2020
Asset Retirement Obligation Disclosure [Abstract]  
Schedule of changes in asset retirement obligations Changes in ARO are as follows:
 
Six Months Ended June 30,
(millions)
2020
 
2019
Asset Retirement Obligations, Beginning Balance
$
814

 
$
880

Liabilities Incurred
19

 
15

Liabilities Settled
(42
)
 
(56
)
Revisions of Estimates
9

 
(70
)
Accretion Expense
21

 
23

Asset Retirement Obligations, Ending Balance
$
821

 
$
792