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Accrued Capping, Closure and Post Closure (Tables) | 12 Months Ended
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Apr. 30, 2013
Changes to Accrued Capping, Closure and Post-Closure Liabilities | The changes to accrued capping, closure and post-closure liabilities are as follows:
|  | Fiscal Year Ended April 30, |
|  | 2013 |  |  | 2012 |
Beginning balance |  | $ | 39,629 |  |  | $ | 36,407 |
Obligations incurred |  |  | 3,188 |  |  |  | 3,123 |
Revisions in estimates (1) |  |  | (694 | ) |  |  | (1,682 | )
Accretion expense |  |  | 3,538 |  |  |  | 3,341 |
Payments |  |  | (2,491 | ) |  |  | (1,560 | )
Ending balance |  | $ | 43,170 |  |  | $ | 39,629 |
(1) | The revisions in estimates for capping, closure and post-closure for the years ended April 30, 2013 and 2012 consist of changes in cost estimates and timing of capping and closure events as well as changes to expansion airspace and tonnage placement assumptions.

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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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