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Asset Retirement Obligations (ARO) (Tables) | 12 Months Ended
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Dec. 31, 2020
Asset Retirement Obligation Disclosure [Abstract] |
Reconciliation of Beginning and Ending Aggregate Carrying Amount of Asset Retirement Obligation | A reconciliation of the beginning and ending aggregate carrying amount of the ARO is shown in the following table.
|  | December 31,
(Millions of dollars) |  | 2020 |  | 2019
Balance at beginning of period |  | $ | 32.8 |  |  | $ | 30.7 |
Accretion expense |  | 2.3 |  |  | 2.1 |
Settlement of liabilities |  | (0.8) |  |  | (0.4) |
Liabilities incurred |  | 0.8 |  |  | 0.4 |
Balance at end of period |  | $ | 35.1 |  |  | $ | 32.8 |

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- Definition Tabular disclosure of the changes in carrying amount of a liability for asset retirement obligations, for changes such as new obligations, changes in estimates of existing obligations, spending on existing obligations, property dispositions, and foreign currency translation. + References Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef -Publisher FASB -Name Accounting Standards Codification -Topic 410 -SubTopic 30 -Section 50 -Paragraph 5 -URI http://asc.fasb.org/extlink&oid=6393242&loc=d3e13201-110859 + Details
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