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Asset Retirement Obligations (Tables)
 12 Months Ended
Dec. 31, 2019
Asset Retirement Obligation Disclosure [Abstract]  
Schedule of Reconciliation of Beginning and Ending Aggregate Carrying Amount of Asset Retirement Obligations
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.