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Commitments and Contingencies - Additional Information (Detail) (USD $) In Thousands, unless otherwise specified | 12 Months Ended | 3 Months Ended | 12 Months Ended |  |  |  | 12 Months Ended
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Apr. 30, 2014 Executives | Apr. 30, 2013 | Apr. 30, 2012 | 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] | Apr. 30, 2014 Potsdam Environmental Liability [Member] acre | Apr. 30, 2013 Potsdam Environmental Liability [Member] | May 31, 2009 Potsdam Environmental Liability [Member] | Sep. 30, 2011 Potsdam Environmental Liability [Member] Waste-Stream Inc [Member] | Nov. 30, 2011 Potsdam Environmental Liability [Member] Motors Liquidation Trust [Member] | Apr. 30, 2014 Southbridge Landfill [Member] Condition One [Member] | Apr. 30, 2014 Maximum [Member] | Apr. 30, 2014 Maximum [Member] Employment Contracts [Member] | Apr. 30, 2014 Minimum [Member] | Apr. 30, 2014 Minimum [Member] Employment Contracts [Member]
Contingencies And Commitments [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Operating facilities and equipment under operating leases |  |  |  |  |  |  |  |  |  |  |  |  |  | $ 26 |  |  |
Total rent expense under operating leases charged to operations | 5,651 | 5,372 | 5,213 |  |  |  |  |  |  |  |  |  |  |  |  |  |
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 | 400 |  |  | 549 | 335 | 1,532 | 2,823 |  |  |  |  |  | 400 |  |  |  |
Percentage of environmental remediation cost |  |  |  |  |  | 15.00% | 30.00% |  |  |  |  |  |  |  |  |  |
Environmental remediation liability | 5,320 |  |  |  |  |  |  | 5,320 | 5,297 |  |  |  |  |  |  |  |
Accretion expense |  |  |  |  |  |  |  | 19 | 138 |  |  |  |  |  |  |  |
Estimate of inflation rate | 2.60% | 2.70% |  |  |  |  |  | 2.00% |  |  |  |  |  |  |  |  |
Future remediation costs |  |  |  |  |  |  |  |  |  |  | 3,000 | 100 |  |  |  |  |
Anticipated remediation costs |  |  |  |  |  |  |  |  |  |  |  |  | 2,100 |  |  |  |
Loss contingency range of possible loss minimum |  |  |  |  |  |  |  |  |  |  |  |  | 400 |  |  |  |
Loss contingency range of possible loss maximum |  |  |  |  |  |  |  |  |  |  |  |  | 2,100 |  |  |  |
Number of executive officers | 4 |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Initial term of employment contract |  |  |  |  |  |  |  |  |  |  |  |  |  | 3 years |  | 1 year |
Intangible assets, useful life |  |  |  |  |  |  |  |  |  |  |  |  |  | 10 years | 2 years | 5 years | 1 year
Contract extension period | 1 year |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Total annual commitments for salaries | $ 1,397 |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Salary and bonuses payment range |  |  |  |  |  |  |  |  |  |  |  |  |  | 3 years |  | 1 year |

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