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Asset Retirement Obligations (Tables) | 12 Months Ended
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Dec. 31, 2020
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 2020 and 2019 is shown in the following table.
(Thousands of dollars) | 2020 |  | 2019
Balance at beginning of year | $ | 865,109 |  |  | 800,117 |
Accretion expense | 42,136 |  |  | 40,506 |
Liabilities incurred | 14,736 |  |  | 14,759 |
Liabilities assumed from acquisitions | — |  |  | 64,810 |
Revisions of previous estimates | (70,098) |  |  | (34,371) |
Liabilities settled | (4,816) |  |  | (25,544) |
Liabilities associated with assets held for sale | (21) |  |  | (240) |
Changes due to translation of foreign currencies | 2,910 |  |  | 5,072 |
Balance at end of year | 849,956 |  |  | 865,109 |
Current portion of liability at end of year ¹ | (33,648) |  |  | (39,315) |
Noncurrent portion of liability at end of year | $ | 816,308 |  |  | 825,794 |
1 Included in Other accrued liabilities on the Consolidated Balance Sheet.

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- Definition Tabular disclosure of the carrying amount of a liability for asset retirement obligations. + References Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef -Publisher FASB -Name Accounting Standards Codification -Topic 410 -SubTopic 20 -Section 50 -Paragraph 1 -URI http://asc.fasb.org/extlink&oid=6392692&loc=d3e7535-110849 + Details
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