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Asset Retirement Obligations
 3 Months Ended
Mar. 31, 2014
Asset Retirement Obligation Disclosure [Abstract]  
Asset Retirement Obligations
Asset Retirement Obligations
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:
 
Three Months Ended
March 31,
(millions)
2014
 
2013
Asset Retirement Obligations, Beginning Balance
$
586

 
$
402

Liabilities Incurred
1

 
1

Liabilities Settled
(14
)
 
(10
)
Revision of Estimate
72

 
(1
)
Accretion Expense (1)
10

 
7

Asset Retirement Obligations, Ending Balance
$
655

 
$
399


(1) Accretion expense is included in DD&A expense in the consolidated statements of operations.
Liabilities settled in 2014 include $24 million for onshore US and deepwater Gulf of Mexico abandonments and $17 million related to properties classified as held for sale, offset by $27 million as a result of reclassifying remaining North Sea assets from held for sale to held and used.
Liabilities settled in 2013 relate primarily to non-core onshore US properties that were sold. See Note 3. Divestitures.
Revision in estimate for 2014 includes an increase of $67 million related to a non-operated North Sea field due to an increase in costs and a change in timing. See Note 4. Asset Impairments.