Contingencies - Additional Information (Detail) (USD $) In Thousands, unless otherwise specified	3 Months Ended						6 Months Ended					3 Months Ended	6 Months Ended
	Oct. 31, 2013	Oct. 31, 2012	Apr. 30, 2011	Apr. 30, 2010	Apr. 30, 2009	Jan. 31, 2009	Oct. 31, 2013	Oct. 31, 2012	Apr. 30, 2013	May 31, 2009	Dec. 20, 2000 acre	Oct. 31, 2013 Southbridge Landfill [Member]	Oct. 31, 2013 Southbridge Landfill [Member]	Oct. 31, 2013 Southbridge Landfill [Member] Minimum [Member]	Oct. 31, 2013 Southbridge Landfill [Member] Maximum [Member]	Sep. 30, 2011 Waste-Stream Inc [Member]	Nov. 30, 2011 Motors Liquidation Trust [Member]
Contingencies And Commitments [Line Items]
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
Environmental remediation charge	150		549	335	1,532	2,823	150					150		500	1,000
Percentage of environmental remediation cost					15.00%	30.00%
Environmental remediation liability	5,397						5,397		5,297
Accretion expense	34	34					69	69
Percentage of risk free interest rate	2.70%						2.70%		2.70%
Future remediation costs																3,000	100
Loss in range after considering amounts for reimbursement, low estimate													150
Loss in range after considering amounts for reimbursement, high estimate													$ 400