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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
	Dec. 31, 2014	Apr. 30, 2014	Apr. 30, 2013
Asset Retirement Obligation Disclosure [Abstract]
Beginning balance	$ 44,654us-gaap_AssetRetirementObligation	$ 43,170us-gaap_AssetRetirementObligation	$ 39,629us-gaap_AssetRetirementObligation
Obligations incurred	2,169us-gaap_AssetRetirementObligationLiabilitiesIncurred	3,621us-gaap_AssetRetirementObligationLiabilitiesIncurred	3,188us-gaap_AssetRetirementObligationLiabilitiesIncurred
Revisions in estimates	(3,378)us-gaap_AssetRetirementObligationRevisionOfEstimate	(3,728)us-gaap_AssetRetirementObligationRevisionOfEstimate	(694)us-gaap_AssetRetirementObligationRevisionOfEstimate
Accretion expense	2,275us-gaap_AssetRetirementObligationAccretionExpense	3,967us-gaap_AssetRetirementObligationAccretionExpense	3,538us-gaap_AssetRetirementObligationAccretionExpense
Obligations settled	(5,891)us-gaap_AssetRetirementObligationLiabilitiesSettled	(2,376)us-gaap_AssetRetirementObligationLiabilitiesSettled	(2,491)us-gaap_AssetRetirementObligationLiabilitiesSettled
Ending balance	$ 39,829us-gaap_AssetRetirementObligation	$ 44,654us-gaap_AssetRetirementObligation	$ 43,170us-gaap_AssetRetirementObligation

X

- 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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Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	instant

X

- 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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Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	duration

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- Details

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Namespace Prefix:	us-gaap_
Data Type:	xbrli:stringItemType
Balance Type:	na
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X

- 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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Namespace Prefix:	us-gaap_
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Balance Type:	credit
Period Type:	duration

X

- 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

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Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	duration

X

- 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

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Period Type:	duration