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Asset Retirement Obligations
 12 Months Ended
Dec. 31, 2019
Asset Retirement Obligation Disclosure [Abstract]  
Asset Retirement Obligations Asset Retirement Obligations
The asset retirement obligations liabilities (ARO) recognized by the Company at December 31, 2019 and 2018 are related to the estimated costs to dismantle and abandon its producing oil and natural gas properties and related equipment.
A reconciliation of the beginning and ending aggregate carrying amount of the asset retirement obligation for 2019 and 2018 is shown in the following table.
(Thousands of dollars)
2019
 
2018
Balance at beginning of year
$
800,117

 
447,270

Accretion expense
40,506

 
27,119

Liabilities incurred
14,759

 
6,572

Liabilities assumed from acquisitions
64,810

 
359,643

Revisions of previous estimates
(34,371
)
 
(20,012
)
Liabilities settled
(25,544
)
 
(11,510
)
Liabilities associated with assets held for sale
(240
)
 
—

Changes due to translation of foreign currencies
5,072

 
(8,965
)
Balance at end of year
865,109

 
800,117

Current portion of liability at end of year ¹
(39,315
)
 
(47,598
)
Noncurrent portion of liability at end of year
$
825,794

 
752,519

1 Included in Other accrued liabilities on the Consolidated Balance Sheet.
The estimation of future ARO is based on a number of assumptions requiring professional judgment.  The Company cannot predict the type of revisions to these assumptions that may be required in future periods due to the availability of additional information such as:  prices for oil field services, technological changes, governmental requirements and other factors.
Liabilities assumed in 2019, primarily represent obligations assumed as part of the LLOG acquisition. Liabilities assumed in 2018 primarily represent obligations from the MP GOM acquisition. (see Note D).