Contingencies (Narrative) (Detail) (USD $) In Thousands, unless otherwise specified	1 Months Ended		3 Months Ended									1 Months Ended
	Apr. 30, 2011	May 31, 2009	Jul. 31, 2012	Jul. 31, 2011	Apr. 30, 2011	Apr. 30, 2010	Apr. 30, 2009	Jan. 31, 2009	May 31, 2012	Jan. 18, 2011	Dec. 20, 2000 acre	Sep. 30, 2011 Waste Stream Inc [Member]	Nov. 30, 2011 Motors Liquidation Trust [Member]
Contingencies And Commitments [Line Items]
Disputed invoice amount			$ 3,195						$ 3,195
Range of possible loss minimum			0
Aggregate finance asserting liability total										3,852
Taxation payable										2,220
Penalties and interest payable										1,632
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					549	335	1,532	2,823				3,000	100
Percentage of environmental remediation cost							15.00%	30.00%
Environmental remediation liability	5,210		5,244		5,210
Accretion expense			$ 34	$ 34
Percentage of risk free interest rate	2.70%				2.70%