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Asset Retirement Obligations (Tables) | 12 Months Ended
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Dec. 31, 2016
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
| 2016 |  | 2015
Asset retirement obligation at beginning of period | $ | 26.2 |  |  | $ | 37.0 |
Accretion expense | 2.3 |  |  | 2.1 |
Payments | (2.2 | ) |  | (1.6 | )
Revisions in estimated cash flows | 1.2 |  |  | 0.2 |
Liabilities incurred | 0.5 |  |  | (0.3 | )
Sale of St. Helens (a) | — |  |  | (11.2 | )
Asset retirement obligation at end of period | $ | 28.0 |  |  | $ | 26.2 |

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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://www.xbrl.org/2003/role/presentationRef -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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