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Asset Retirement Obligations | 12 Months Ended
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Dec. 31, 2019
Asset Retirement Obligation Disclosure [Abstract] |
Asset Retirement Obligations | Asset Retirement Obligations The reconciliation of the changes in our AROs is presented below:
| Year ended December 31,
| 2019 |  | 2018
Beginning balance, ARO liability | $ | 18.2 |  |  | $ | 28.5 |
Additional ARO liability incurred | 2.9 |  |  | 3.6 |
ARO liabilities settled | (3.5 | ) |  | (6.5 | )
Accretion expense | 1.3 |  |  | 1.9 |
Revisions in estimated cash flows | — |  |  | (9.3 | )
Ending balance, ARO liability | $ | 18.9 |  |  | $ | 18.2 |
Revisions in estimated cash flows during 2018 were related to a change in estimate of future disposal costs of spent potliners.

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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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