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Asset Retirement Obligations | 12 Months Ended
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Dec. 31, 2020
Asset Retirement Obligation Disclosure [Abstract] |
Asset Retirement Obligations | Asset Retirement Obligations Reconciliations of the Company’s asset retirement obligations are as follows:
| December 31,
| 2020 |  | 2019
| (Dollars in millions)
Balance at beginning of period | $ | 752.3 |  |  | $ | 750.2 |
Liabilities settled or disposed | (38.4) |  |  | (47.7) |
Accretion expense | 56.0 |  |  | 54.1 |
Revisions to estimates | (41.7) |  |  | (4.3) |
Balance at end of period | $ | 728.2 |  |  | $ | 752.3 |
Less: Current portion (included in “Accounts payable and accrued expenses”) | 77.7 |  |  | 98.2 |
Noncurrent obligation (included in “Asset retirement obligations”) | $ | 650.5 |  |  | $ | 654.1 |
Balance at end of period — active locations | $ | 471.8 |  |  | $ | 525.4 |
Balance at end of period — closed or inactive locations | $ | 256.4 |  |  | $ | 226.9 |
The credit-adjusted, risk-free interest rates utilized to estimate the Company’s asset retirement obligations ranged from 9.16% for life of mines 3 years or less to 12.74% for life of mines greater than 20 years for both U.S. and Australia reclamation obligations at December 31, 2020 and ranged from 9.24% for life of mines 3 years or less to 12.38% for life of mines greater than 20 years for both U.S. and Australia reclamation obligations at December 31, 2019.As of December 31, 2020 and 2019, the Company had $1,451.9 million and $1,401.7 million, respectively, in surety bonds outstanding to secure reclamation obligations. Additionally, the Company had $315.0 million and $106.1 million, respectively, of letters of credit in support of reclamation obligations as of December 31, 2020 and 2019.

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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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