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Commitments and Contingencies - Reconciliation of Expected Aggregate Undiscounted Amount to Amount Recognized in Statements of Financial Position (Detail) (USD $) In Thousands, unless otherwise specified	Apr. 30, 2013	Apr. 30, 2012
Environmental Liabilities [Line Items]
Undiscounted liability	$ 5,072
Plus inflation / discount	225
Liability balance	$ 5,297	$ 5,210

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- Definition

Amount of imputed interest necessary to reduce (reconcile) the undiscounted amount of the accrual for environmental loss contingencies to the present value.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef

-Publisher FASB

-Name Accounting Standards Codification

-Topic 450

-SubTopic 20

-Section S99

-Paragraph 1

-Subparagraph (SAB TOPIC 5.Y.Q1)

-URI http://asc.fasb.org/extlink&oid=6572457&loc=d3e149879-122751



Reference 2: http://www.xbrl.org/2003/role/presentationRef

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-Name Staff Accounting Bulletin (SAB)

-Number Topic 5

-Section Y

-Paragraph Question 1

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- Definition

Undiscounted amount of the accrual for environmental loss contingencies.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef

-Publisher AICPA

-Name Statement of Position (SOP)

-Number 96-1

-Paragraph 161

-Subparagraph a

-LegacyDoc This reference is SUPERSEDED by the Accounting Standards Codification effective for interim and annual periods ending after September 15, 2009. This reference is included to help users transition from the previous accounting hierarchy and will be removed from future versions of this taxonomy.



Reference 2: http://www.xbrl.org/2003/role/presentationRef

-Publisher FASB

-Name Accounting Standards Codification

-Topic 450

-SubTopic 20

-Section S99

-Paragraph 1

-Subparagraph (SAB TOPIC 5.Y.Q1)

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Reference 4: http://www.xbrl.org/2003/role/presentationRef

-Publisher FASB

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-Topic 410

-SubTopic 30

-Section 50

-Paragraph 7

-URI http://asc.fasb.org/extlink&oid=6393242&loc=d3e13207-110859

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- Definition

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+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef

-Publisher AICPA

-Name Statement of Position (SOP)

-Number 96-1

-Paragraph 161, 132

-LegacyDoc This reference is SUPERSEDED by the Accounting Standards Codification effective for interim and annual periods ending after September 15, 2009. This reference is included to help users transition from the previous accounting hierarchy and will be removed from future versions of this taxonomy.



Reference 2: http://www.xbrl.org/2003/role/presentationRef

-Publisher FASB

-Name Accounting Standards Codification

-Topic 450

-SubTopic 20

-Section S99

-Paragraph 1

-Subparagraph (SAB TOPIC 5.Y.Q1)

-URI http://asc.fasb.org/extlink&oid=6572457&loc=d3e149879-122751



Reference 3: http://www.xbrl.org/2003/role/presentationRef

-Publisher SEC

-Name Staff Accounting Bulletin (SAB)

-Number Topic 5

-Section Y

-Paragraph Question 1



Reference 4: http://www.xbrl.org/2003/role/presentationRef

-Publisher FASB

-Name Accounting Standards Codification

-Topic 410

-SubTopic 30

-Section 50

-Paragraph 7

-URI http://asc.fasb.org/extlink&oid=6393242&loc=d3e13207-110859

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