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Contingencies - Additional Information (Detail) (USD $) In Thousands, unless otherwise specified | 3 Months Ended | 9 Months Ended | 3 Months Ended | 9 Months Ended |  |  |  |  |  | 3 Months Ended | 9 Months Ended
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Jan. 31, 2014 | Jan. 31, 2014 | Jan. 31, 2014 Potsdam Environmental Liability [Member] | Jan. 31, 2013 Potsdam Environmental Liability [Member] | Apr. 30, 2011 Potsdam Environmental Liability [Member] | Apr. 30, 2010 Potsdam Environmental Liability [Member] | Apr. 30, 2009 Potsdam Environmental Liability [Member] | Jan. 31, 2009 Potsdam Environmental Liability [Member] | Jan. 31, 2014 Potsdam Environmental Liability [Member] | Jan. 31, 2013 Potsdam Environmental Liability [Member] | Apr. 30, 2013 Potsdam Environmental Liability [Member] | May 31, 2009 Potsdam Environmental Liability [Member] | Dec. 20, 2000 Potsdam Environmental Liability [Member] acre | Sep. 30, 2011 Potsdam Environmental Liability [Member] Waste-Stream Inc [Member] | Nov. 30, 2011 Potsdam Environmental Liability [Member] Motors Liquidation Trust [Member] | Jan. 31, 2014 Southbridge Landfill [Member] Condition One [Member] | Jan. 31, 2014 Southbridge Landfill [Member] Condition One [Member]
Contingencies And Commitments [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Scrap yard and solid waste transfer station |  |  |  |  |  |  |  |  |  |  |  |  | 25 |  |  |  |
Estimate of total undiscounted costs associated with implementing the preferred remedies |  |  |  |  | $ 12,130 |  |  |  |  |  |  | $ 10,219 |  |  |  |  |
Environmental remediation charge | 250 | 400 |  |  | 549 | 335 | 1,532 | 2,823 |  |  |  |  |  |  |  | 250 | 400
Percentage of environmental remediation cost |  |  |  |  |  |  | 15.00% | 30.00% |  |  |  |  |  |  |  |  |
Environmental remediation liability |  |  | 5,406 |  |  |  |  |  | 5,406 |  | 5,297 |  |  |  |  |  |
Accretion expense |  |  | 34 | 34 |  |  |  |  | 103 | 103 |  |  |  |  |  |  |
Percentage of risk free interest rate |  |  | 2.70% |  |  |  |  |  | 2.70% |  | 2.70% |  |  |  |  |  |
Future remediation costs |  |  |  |  |  |  |  |  |  |  |  |  |  | 3,000 | 100 |  |
Anticipated remediation costs |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  | 2,100
Loss contingency range of possible loss minimum | 0 | 0 |  |  |  |  |  |  |  |  |  |  |  |  |  | 400 | 400
Loss contingency range of possible loss maximum | $ 864 | $ 864 |  |  |  |  |  |  |  |  |  |  |  |  |  | $ 2,100 | $ 2,100

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- Definition The face value of future remediation costs to be paid by GM. + References No definition available. + Details
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- Definition Percentage, or additional percentage, of estimated remediation costs that we are responsible for. + References No definition available. + Details
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- Definition Acreage of scrap yard owned by WSI. + References No definition available. + Details
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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 Rate applied to the undiscounted amount of environmental loss contingencies to arrive at the present value recorded as of the balance sheet date. + 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=27011672&loc=d3e149879-122751 Reference 2: http://www.xbrl.org/2003/role/presentationRef -Publisher SEC -Name Staff Accounting Bulletin (SAB) -Number Topic 5 -Section Y -Paragraph Question 1 Reference 3: 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 + Details
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