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Asset Retirement Obligations (Tables)
 12 Months Ended
Dec. 31, 2018
Asset Retirement Obligation Disclosure [Abstract]  
Schedule of Change in Asset Retirement Obligation ARO consists primarily of estimated costs of dismantlement, removal, site reclamation and similar activities associated with our oil and gas properties. Changes in ARO are as follows:
 
Year Ended December 31,
(millions)
2018
 
2017
Asset Retirement Obligations, Beginning Balance
$
875

 
$
935

Liabilities Incurred
25

 
94

Liabilities Settled
(345
)
 
(82
)
Revisions of Estimates
293

 
(65
)
Reclassification to Liabilities Associated with Assets Held for Sale
(1
)
 
(54
)
Accretion Expense
33

 
47

Asset Retirement Obligations, Ending Balance
$
880

 
$
875