Segment Reporting (Summary Of Financial Information By Reportable Segment) (Detail) (USD $) In Thousands, unless otherwise specified	3 Months Ended
	Jul. 31, 2012		Jul. 31, 2011		Apr. 30, 2012
Segment Reporting Information [Line Items]
Outside revenues	$ 121,195		$ 127,193
Inter-company revenue		[1]		[1]
Depreciation and amortization	14,756		14,506
Operating income (loss)	5,751		10,256
Total assets	643,274		694,872		633,743
Eastern Region [Member]
Segment Reporting Information [Line Items]
Outside revenues	46,200		44,759
Inter-company revenue	8,253	[1]	9,750	[1]
Depreciation and amortization	6,445		5,960
Operating income (loss)	(203)		215
Total assets	175,252		218,814
Western Region [Member]
Segment Reporting Information [Line Items]
Outside revenues	53,367		56,872
Inter-company revenue	17,456	[1]	19,271	[1]
Depreciation and amortization	6,777		7,148
Operating income (loss)	6,383		9,348
Total assets	352,068		343,572
Recycling [Member]
Segment Reporting Information [Line Items]
Outside revenues	10,365		12,903
Inter-company revenue	(50)	[1]	(46)	[1]
Depreciation and amortization	1,068		928
Operating income (loss)	42		2,248
Total assets	53,924		58,335
Other [Member]
Segment Reporting Information [Line Items]
Outside revenues	11,263		12,659
Inter-company revenue	775	[1]	462	[1]
Depreciation and amortization	466		470
Operating income (loss)	(471)		(1,555)
Total assets	62,030		74,151
Elimination [Member]
Segment Reporting Information [Line Items]
Outside revenues
Inter-company revenue	(26,434)	[1]	(29,437)	[1]
Depreciation and amortization
Operating income (loss)
Total assets

[1] Inter-company revenues reflect transactions with and between segments that are generally made on a basis intended to reflect the market value of such services.