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Final Capping, Closure and Post-Closure Costs - Changes to Accrued Capping, Closure and Post-Closure Liabilities (Detail) (USD $) In Thousands, unless otherwise specified | 8 Months Ended | 12 Months Ended
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Dec. 31, 2014 | Apr. 30, 2014 | Apr. 30, 2013
Asset Retirement Obligation Disclosure [Abstract] |  |  |
Beginning balance | $ 44,654 us-gaap_AssetRetirementObligation | $ 43,170 us-gaap_AssetRetirementObligation | $ 39,629 us-gaap_AssetRetirementObligation
Obligations incurred | 2,169 us-gaap_AssetRetirementObligationLiabilitiesIncurred | 3,621 us-gaap_AssetRetirementObligationLiabilitiesIncurred | 3,188 us-gaap_AssetRetirementObligationLiabilitiesIncurred
Revisions in estimates | (3,378)us-gaap_AssetRetirementObligationRevisionOfEstimate | (3,728)us-gaap_AssetRetirementObligationRevisionOfEstimate | (694)us-gaap_AssetRetirementObligationRevisionOfEstimate
Accretion expense | 2,275 us-gaap_AssetRetirementObligationAccretionExpense | 3,967 us-gaap_AssetRetirementObligationAccretionExpense | 3,538 us-gaap_AssetRetirementObligationAccretionExpense
Obligations settled | (5,891)us-gaap_AssetRetirementObligationLiabilitiesSettled | (2,376)us-gaap_AssetRetirementObligationLiabilitiesSettled | (2,491)us-gaap_AssetRetirementObligationLiabilitiesSettled
Ending balance | $ 39,829 us-gaap_AssetRetirementObligation | $ 44,654 us-gaap_AssetRetirementObligation | $ 43,170 us-gaap_AssetRetirementObligation

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- Definition The carrying amount of a liability for an asset retirement obligation. An asset retirement obligation is a legal obligation associated with the disposal or retirement 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 -Section 50 -Paragraph 1 -Subparagraph (c) -URI http://asc.fasb.org/extlink&oid=6392692&loc=d3e7535-110849 Reference 2: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Glossary Asset Retirement Obligation -URI http://asc.fasb.org/extlink&oid=6505190 + Details
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- Definition Amount of accretion expense recognized during the period that is associated with an asset retirement obligation. Accretion expense measures and incorporates changes due to the passage of time into the carrying amount of the liability. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 410 -SubTopic 20 -Section 45 -Paragraph 1 -URI http://asc.fasb.org/extlink&oid=6392676&loc=d3e7480-110848 Reference 2: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 410 -SubTopic 20 -Section 50 -Paragraph 1 -Subparagraph (c)(3) -URI http://asc.fasb.org/extlink&oid=6392692&loc=d3e7535-110849 + Details
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- Details
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- Definition Amount of asset retirement obligations incurred during the period. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 410 -SubTopic 20 -Section 50 -Paragraph 1 -Subparagraph (c)(1) -URI http://asc.fasb.org/extlink&oid=6392692&loc=d3e7535-110849 + Details
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- Definition Amount of asset retirement obligations settled, or otherwise disposed of, during the period. This may include asset retirement obligations transferred to third parties associated with the sale of a long-lived asset. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 410 -SubTopic 20 -Section 50 -Paragraph 1 -Subparagraph (c)(2) -URI http://asc.fasb.org/extlink&oid=6392692&loc=d3e7535-110849 + Details
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- Definition Amount of the Increase or Decrease in the amount of the asset retirement obligation during the current period for changes in the amount or timing of the estimated cash flows associated with the settlement of the obligation. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 410 -SubTopic 20 -Section 50 -Paragraph 1 -Subparagraph (c)(4) -URI http://asc.fasb.org/extlink&oid=6392692&loc=d3e7535-110849 + Details
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