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Asset Retirement Obligations
 3 Months Ended
Mar. 31, 2017
Asset Retirement Obligation Disclosure [Abstract]  
Asset Retirement Obligations
Asset Retirement Obligations
Asset retirement obligations (ARO) consist 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:
 
Three Months Ended
March 31,
(millions)
2017
 
2016
Asset Retirement Obligations, Beginning Balance
$
935

 
$
989

Liabilities Incurred
1

 
2

Liabilities Settled
(9
)
 
(8
)
Revision of Estimate
(7
)
 
5

Accretion Expense (1)
12

 
12

Asset Retirement Obligations, Ending Balance
$
932

 
$
1,000

(1) 
Accretion expense is included in depreciation, depletion and amortization (DD&A) expense in the consolidated statements of operations.
For the three months ended March 31, 2017 Liabilities incurred were due to new wells and facilities placed into service for US onshore. Liabilities settled primarily related to US onshore property abandonments. Revisions of estimates related to changes in cost estimates of $7 million for deepwater Gulf of Mexico.
For the three months ended March 31, 2016 Liabilities incurred were due to new wells and facilities for US onshore. Liabilities settled primarily related to US onshore property abandonments. Revisions of estimates relate to changes in cost estimates of $5 million for Equatorial Guinea.