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Asset Retirement Obligations
 3 Months Ended
Mar. 31, 2018
Asset Retirement Obligation Disclosure [Abstract]  
Asset Retirement Obligations Note 8. 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)
2018
 
2017
Asset Retirement Obligations, Beginning Balance
$
875

 
$
935

Liabilities Incurred
2

 
1

Liabilities Settled
(20
)
 
(9
)
Revisions of Estimates
(11
)
 
(7
)
Reclassification to Liabilities Associated with Assets Held for Sale
(227
)
 
—

Accretion Expense (1)
9

 
12

Asset Retirement Obligations, Ending Balance
$
628

 
$
932


(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, 2018 In 2018, we transferred $227 million of ARO liabilities related to Gulf of Mexico properties to liabilities associated with assets held for sale. Liabilities settled include $20 million related to abandonment of US onshore properties, primarily in the DJ Basin. Revisions of estimates relate to decreases in cost and timing estimates of $11 million associated with the North Sea abandonment project.
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 Gulf of Mexico cost estimates.