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Asset Retirement Obligations (ARO) | 3 Months Ended
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Mar. 31, 2021
Asset Retirement Obligation Disclosure [Abstract] |
Asset Retirement Obligations (ARO) | Asset Retirement Obligations (ARO) The majority of the ARO recognized by the Company at March 31, 2021 and December 31, 2020 is related to the estimated costs to dismantle and abandon certain of its retail gasoline stations. The Company has not recorded an ARO for certain of its marketing assets because sufficient information is presently not available to estimate a range of potential settlement dates for the obligation. These assets are consistently being upgraded and are expected to be operational into the foreseeable future. In these cases, the obligation will be initially recognized in the period in which sufficient information exists to estimate the obligation. A reconciliation of the beginning and ending aggregate carrying amount of the ARO is shown in the following table.
(Millions of dollars) |  | March 31, 2021 |  | December 31, 2020
Balance at beginning of period |  | $ | 35.1 |  |  | $ | 32.8 |
Addition for acquisition |  | 1.2 |  |  | — |
Accretion expense |  | 0.6 |  |  | 2.3 |
Settlements of liabilities |  | (0.2) |  |  | (0.8) |
Liabilities incurred |  | — |  |  | 0.8 |
Balance at end of period |  | $ | 36.7 |  |  | $ | 35.1 |
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 lack of availability of additional information.

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- References No definition available. + Details
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- Definition The entire disclosure for an asset retirement obligation and the associated long-lived asset. An asset retirement obligation is a legal obligation associated with the disposal or retirement from service of a tangible long-lived asset that results from the acquisition, construction or development, or the normal operations of a long-lived asset, except for certain obligations of lessees. + References Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef -Publisher FASB -Name Accounting Standards Codification -Topic 410 -SubTopic 20 -URI http://asc.fasb.org/subtopic&trid=2175671 + Details
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