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Asset Retirement Obligations (Tables) | 12 Months Ended
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Dec. 31, 2020
Asset Retirement Obligation [Abstract] |
Changes to Asset Retirement Obligation | The following table describes changes to the asset retirement obligation liability (dollars in millions):
|  | Year Ended December 31, |
|  | 2020 |  |  | 2019 |
Asset retirement obligation at beginning of period |  | $ | 27.8 |  |  | $ | 30.0 |
Accretion expense |  |  | 1.3 |  |  |  | 1.5 |
Payments |  |  | (0.8 | ) |  |  | (3.5 | )
Revisions in estimated cash flows (a) |  |  | 3.5 |  |  |  | (0.2 | )
Asset retirement obligation at end of period |  | $ | 31.8 |  |  | $ | 27.8 |

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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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