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Asset Retirement Obligations (Tables)
 9 Months Ended
Sep. 30, 2019
Asset Retirement Obligation Disclosure [Abstract]  
Changes in Asset Retirement Obligations Changes in ARO are as follows:
 
Nine Months Ended September 30,
(millions)
2019
 
2018
Asset Retirement Obligations, Beginning Balance
$
880

 
$
875

Liabilities Incurred
17

 
16

Liabilities Settled
(82
)
 
(309
)
Revisions of Estimates
(60
)
 
67

Accretion Expense
33

 
25

Asset Retirement Obligations, Ending Balance
$
788

 
$
674