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Asset Retirement Obligations (Tables)
 3 Months Ended
Mar. 31, 2020
Asset Retirement Obligation Disclosure [Abstract]  
Changes in Asset Retirement Obligations Changes in ARO are as follows:
 
Three Months Ended March 31,
(millions)
2020
 
2019
Asset Retirement Obligations, Beginning Balance
$
814

 
$
880

Liabilities Incurred
3

 
2

Liabilities Settled
(27
)
 
(27
)
Revisions of Estimates
(8
)
 
—

Accretion Expense
11

 
12

Asset Retirement Obligations, Ending Balance
$
793

 
$
867