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Asset Retirement Obligations | 12 Months Ended
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Dec. 31, 2017
Asset Retirement Obligation Disclosure [Abstract] |
Asset Retirement Obligations | Asset Retirement Obligations Reconciliations of the Company’s asset retirement obligations are as follows:
| Successor | Predecessor
| April 2 through December 31, 2017 | January 1 through April 1, 2017 |  | Year Ended December 31, 2016
| (Dollars in millions)
Balance at beginning of period | $ | 664.2 |  | $ | 758.8 |  |  | $ | 712.1 |
Liabilities settled or disposed | (65.2 | ) | (2.7 | ) |  | (41.5 | )
Accretion expense | 32.6 |  | 12.5 |  |  | 45.7 |
Revisions to estimates | 59.5 |  | (104.4 | ) |  | 42.5 |
Balance at end of period | $ | 691.1 |  | $ | 664.2 |  |  | $ | 758.8 |
Less: Current portion (included in “Accounts payable and accrued expenses”) | 34.1 |  | 31.1 |  |  | 41.0 |
Noncurrent obligation (included in “Asset retirement obligations”) | $ | 657.0 |  | $ | 633.1 |  |  | $ | 717.8 |
Balance at end of period — active locations | $ | 612.9 |  | $ | 540.1 |  |  | $ | 634.8 |
Balance at end of period — closed or inactive locations | $ | 78.2 |  | $ | 124.1 |  |  | $ | 124.0 |
During the Successor period April 2 through December 31, 2017, the Company sold its Burton Mine and the related asset retirement obligations, as further discussed in Note 21. “Resource Management, Acquisitions and Other Commercial Events.” The changes in mine operations impacted reclamation estimates and are reflected in the asset retirement obligation asset and liability as of December 31, 2017. The credit-adjusted, risk-free interest rates utilized to estimate the Company’s asset retirement obligations were 9.71%, 9.36% and 13.45% for its U.S. reclamation obligations and 4.65%, 4.36% and 4.92% for its Australia reclamation obligations at December 31, 2017, April 1, 2017 and December 31, 2016, respectively. The credit-adjusted, risk-free interest rate at December 31, 2015 was 50.83%. For 2017 and 2016, a distinct rate was developed for Australia due to the amount of cash collateral held in support of the related obligations as of December 31, 2017, April 1, 2017 and December 31, 2016. As of December 31, 2017 and 2016, the Company had $1,136.8 million and $374.3 million, respectively, in surety bonds and bank guarantees outstanding to secure reclamation obligations. The amount of reclamation self-bonding in certain U.S. states in which the Company qualified was $1,094.2 million as of December 31, 2016. Additionally, the Company had $188.5 million and $80.0 million, respectively, of letters of credit in support of reclamation obligations as of December 31, 2017 and 2016. The Company also had restricted cash and cash collateral of $205.2 million and $233.2 million as of December 31, 2017 and 2016, respectively, in support of reclamation obligations.

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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://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 410 -SubTopic 20 -URI http://asc.fasb.org/subtopic&trid=2175671 + Details
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