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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
	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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Anticipated remediation cost.

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- Definition

The face value of future remediation costs to be paid by GM.

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Percentage, or additional percentage, of estimated remediation costs that we are responsible for.

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Acreage of scrap yard owned by WSI.

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Submitted estimate of total undiscounted costs associated with implementing the preferred remedies.

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Amount recognized for the passage of time, typically for liabilities, that have been discounted to their net present values. Excludes accretion associated with asset retirement obligations.

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Total costs accrued as of the balance sheet date for environmental loss contingencies.

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Rate applied to the undiscounted amount of environmental loss contingencies to arrive at the present value recorded as of the balance sheet date.

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The estimated maximum amount of possible loss from the contingency.

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