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Commitments and Contingencies - Schedule of Environmental Liability Associated with Postdam (Detail) (Potsdam Environmental Remediation Liability [Member], USD $) In Thousands, unless otherwise specified | 6 Months Ended |  |  | 6 Months Ended
--- | --- | --- | --- | ---
Oct. 31, 2014 | Oct. 31, 2013 | Apr. 30, 2013 | Oct. 31, 2013 Obligations Incurred [Member] | Oct. 31, 2014 Accretion Expense [Member] | Oct. 31, 2013 Accretion Expense [Member]
Site Contingency [Line Items] |  |  |  |  |  |
Beginning balance | $ 5,320 | $ 5,397 | $ 5,297 |  |  |
Obligations incurred / Accretion expense |  |  |  | 31 | 69 | 69
Payments | (270) |  |  |  |  |
Ending balance | $ 5,119 | $ 5,397 | $ 5,297 |  |  |

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- Definition Total costs accrued as of the balance sheet date for environmental loss contingencies. + References Reference 1: 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 Reference 2: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 450 -SubTopic 20 -Section 50 -Paragraph 1 -URI http://asc.fasb.org/extlink&oid=25496072&loc=d3e14326-108349 Reference 3: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 450 -SubTopic 20 -Section 25 -Paragraph 2 -URI http://asc.fasb.org/extlink&oid=6395460&loc=d3e13647-108346 + Details
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- Definition The aggregate amount disbursed during the reporting period to settle environmental loss contingencies. + 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.Q2) -URI http://asc.fasb.org/extlink&oid=27011672&loc=d3e149879-122751 + Details
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- Definition The amount added to the accrual for newly incurred environmental loss contingencies. These additions include remediation expense and interest accretion. These additions do not relate to changes in the environmental accrual related to a business acquisition or divestiture, to currency movements, or revisions in previously existing estimates. + 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.Q2) -URI http://asc.fasb.org/extlink&oid=27011672&loc=d3e149879-122751 + Details
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