Commitments and Contingencies - Additional Information (Detail) (USD $) In Thousands, unless otherwise specified	3 Months Ended					3 Months Ended												3 Months Ended	12 Months Ended
	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