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Commitments and Contingencies - Additional Information (Detail) (USD $) In Thousands, unless otherwise specified | 3 Months Ended |  | 3 Months Ended |  |  |  |  | 3 Months Ended | 12 Months Ended
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Jul. 31, 2014 | Jul. 31, 2014 New York Attorney General Matter [Member] | Jul. 31, 2014 Greenwood Street Landfill [Member] | Jul. 31, 2014 Greenwood Street Landfill [Member] Administrative Penalty [Member] | Jul. 31, 2014 Minimum [Member] New York Attorney General Matter [Member] | Jul. 31, 2014 General and Administration Costs [Member] New York Attorney General Matter [Member] | Jul. 31, 2014 General and Administration Costs [Member] Greenwood Street Landfill [Member] | Jul. 31, 2014 Potsdam Environmental Remediation Liability [Member] acre | Jul. 31, 2013 Potsdam Environmental Remediation Liability [Member] | Apr. 30, 2011 Potsdam Environmental Remediation Liability [Member] | Apr. 30, 2010 Potsdam Environmental Remediation Liability [Member] | Apr. 30, 2009 Potsdam Environmental Remediation Liability [Member] | Jan. 31, 2009 Potsdam Environmental Remediation Liability [Member] | Apr. 30, 2014 Potsdam Environmental Remediation Liability [Member] | May 31, 2009 Potsdam Environmental Remediation Liability [Member] | Sep. 30, 2011 Potsdam Environmental Remediation Liability [Member] Waste-Stream Inc. [Member] | Nov. 30, 2011 Potsdam Environmental Remediation Liability [Member] Motors Liquidation Trust [Member] | Jul. 31, 2014 Southbridge Landfill [Member] Condition One [Member] | Apr. 30, 2014 Southbridge Landfill [Member] Condition One [Member]
Contingencies And Commitments [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Minimum acquisition value for post acquisition notice |  |  |  |  | $ 350 |  |  |  |  |  |  |  |  |  |  |  |  |  |
Litigation settlement |  | 100 | 67 | 201 |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Litigation settlement charge |  |  |  |  |  | 100 | 200 |  |  |  |  |  |  |  |  |  |  |  |
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 | 75 |  |  |  |  |  |  |  |  | 549 | 335 | 1,532 | 2,823 |  |  |  |  | 75 | 400
Percentage of environmental remediation cost |  |  |  |  |  |  |  |  |  |  |  | 15.00% | 30.00% |  |  |  |  |  |
Environmental remediation liability |  |  |  |  |  |  |  | 5,354 |  |  |  |  |  | 5,320 |  |  |  |  |
Accretion expense |  |  |  |  |  |  |  | 34 | 34 |  |  |  |  |  |  |  |  |  |
Estimate of inflation rate |  |  |  |  |  |  |  | 2.00% |  |  |  |  |  |  |  |  |  |  |
Future remediation costs |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  | 3,000 | 100 |  |
Anticipated remediation costs |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  | 2,322 |
Loss contingency range of possible loss minimum |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  | 475 |
Loss contingency range of possible loss maximum |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  | $ 2,322 |

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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 The acquisition notification limit agreed to with the New York Attorney General as a part of our settlement. + References No definition available. + Details
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