Contingencies - Additional Information (Detail) (USD $) In Thousands, unless otherwise specified	0 Months Ended	1 Months Ended		3 Months Ended								1 Months Ended
	Nov. 13, 2012	Apr. 30, 2011	May 31, 2009	Jul. 31, 2013	Jul. 31, 2012	Apr. 30, 2011	Apr. 30, 2010	Apr. 30, 2009	Jan. 31, 2009	Apr. 30, 2013	Dec. 20, 2000 acre	Sep. 30, 2011 Waste-Stream Inc [Member]	Nov. 30, 2011 Motors Liquidation Trust [Member]
Contingencies And Commitments [Line Items]
Accruals related to outstanding legal proceedings				$ 0
Scrap yard and solid waste transfer station											25
Undiscounted costs associated with implementing preferred remedies			10,219
Present worth cost associated with implementing preferred remedies		12,130
Enter into a Consent Order with DEC	60 days
Environmental remediation charge						549	335	1,532	2,823
Percentage of environmental remediation cost								15.00%	30.00%
Environmental remediation liability				5,363						5,297
Accretion expense				34	34
Percentage of risk free interest rate				2.70%						2.70%
Future remediation costs												$ 3,000	$ 100