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ASSET RETIREMENT OBLIGATION: (Tables) | 3 Months Ended
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Mar. 31, 2019
ASSET RETIREMENT OBLIGATION: |
Summary of asset retirement obligation activity | The following table summarizes the asset retirement obligation activity for the three months ended March 31, 2019 and 2018 (in millions):
|  | 2019 |  | 2018
Balance as of January 1 |  | $ | 217.7 |  | $ | 222.5
Changes in estimates |  |  | 28.1 |  |  | (5.2)
Payments |  |  | (0.2) |  |  | —
Accretion expense |  |  | 3.0 |  |  | 3.2
Balance as of March 31, |  | $ | 248.6 |  | $ | 220.5

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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://fasb.org/us-gaap/role/ref/legacyRef -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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