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Asset Retirement Obligations | 12 Months Ended
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Dec. 31, 2016
Asset Retirement Obligation Disclosure [Abstract] |
Asset Retirement Obligations | Asset Retirement Obligations Reconciliations of the Company’s asset retirement obligations are as follows:
| December 31,
| 2016 |  | 2015
| (Dollars in millions)
Balance at beginning of year | $ | 712.1 |  |  | $ | 752.5 |
Liabilities incurred or acquired | — |  |  | 1.3 |
Liabilities settled or disposed | (41.5 | ) |  | (53.3 | )
Accretion expense | 45.7 |  |  | 42.7 |
Revisions to estimates | 42.5 |  |  | (31.1 | )
Balance at end of year | $ | 758.8 |  |  | $ | 712.1 |
Less: Current portion (included in "Accounts payable and accrued expenses") | 41.0 |  |  | 25.5 |
Noncurrent obligation (included in "Asset retirement obligations") | $ | 717.8 |  |  | $ | 686.6 |
Balance at end of year — active locations | $ | 651.1 |  |  | $ | 656.8 |
Balance at end of year — closed or inactive locations | $ | 107.7 |  |  | $ | 55.3 |
The credit-adjusted, risk-free interest rates utilized to estimate the Company's asset retirement obligations were 13.45% for its U.S. reclamation obligations and 4.92% for its Australia reclamation obligations at December 31, 2016 and 50.83% and 6.82% at December 31, 2015 and 2014, respectively. For 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, 2016. As of December 31, 2016 and 2015, the Company had $374.3 million and $609.4 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 qualifies was $1,094.2 million and $1,430.8 million as of December 31, 2016 and 2015, respectively. Additionally, the Company had $80.0 million and $126.6 million, respectively, of letters of credit in support of reclamation obligations as of December 31, 2016 and 2015. During 2016, the Company replaced certain bank guarantees with cash collateral of $233.2 million as of December 31, 2016.

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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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