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Asset Retirement Obligations (Tables)
 6 Months Ended
Jun. 30, 2019
Asset Retirement Obligation Disclosure [Abstract]  
Changes in Asset Retirement Obligations Changes in ARO are as follows:
 
Six Months Ended June 30,
(millions)
2019
 
2018
Asset Retirement Obligations, Beginning Balance
$
880

 
$
875

Liabilities Incurred
15

 
14

Liabilities Settled
(56
)
 
(261
)
Revisions of Estimates
(70
)
 
(10
)
Accretion Expense
23

 
17

Asset Retirement Obligations, Ending Balance
$
792

 
$
635