Segment Reporting (Summary of Financial Information by Reportable Segment) (Detail) (USD $) In Thousands, unless otherwise specified	3 Months Ended				9 Months Ended
	Jan. 31, 2013		Jan. 31, 2012		Jan. 31, 2013		Jan. 31, 2012		Apr. 30, 2012
Segment Reporting Information [Line Items]
Outside revenues	$ 115,002		$ 114,578		$ 356,531		$ 371,637
Inter-company revenue		[1]		[1]		[1]		[1]
Depreciation and amortization	14,045		14,827		43,433		44,394
Operating income (loss)	(52)		4,420		10,074		26,303
Total assets	673,153		674,369		673,153		674,369		633,743
Eastern Region [Member]
Segment Reporting Information [Line Items]
Outside revenues	43,777		42,208		134,726		132,158
Inter-company revenue	7,553	[1]	7,818	[1]	24,087	[1]	27,111	[1]
Depreciation and amortization	5,699		6,272		18,177		18,301
Operating income (loss)	(3,156)		(975)		(4,299)		(87)
Total assets	206,885		213,058		206,885		213,058
Western Region [Member]
Segment Reporting Information [Line Items]
Outside revenues	51,165		50,746		159,879		167,154
Inter-company revenue	16,729	[1]	16,274	[1]	52,206	[1]	55,395	[1]
Depreciation and amortization	6,989		6,922		20,889		21,652
Operating income (loss)	4,268		5,354		17,762		24,175
Total assets	362,092		345,371		362,092		345,371
Recycling [Member]
Segment Reporting Information [Line Items]
Outside revenues	9,904		10,872		29,308		37,595
Inter-company revenue	(9)	[1]	(75)	[1]	(72)	[1]	(179)	[1]
Depreciation and amortization	1,039		1,074		3,158		2,935
Operating income (loss)	(23)		562		(422)		4,905
Total assets	51,700		56,844		51,700		56,844
Other [Member]
Segment Reporting Information [Line Items]
Outside revenues	10,156		10,752		32,618		34,730
Inter-company revenue	34	[1]	821	[1]	1,300	[1]	1,897	[1]
Depreciation and amortization	318		559		1,209		1,506
Operating income (loss)	(1,141)		(521)		(2,967)		(2,690)
Total assets	52,476		59,096		52,476		59,096
Eliminations [Member]
Segment Reporting Information [Line Items]
Inter-company revenue	$ (24,307)	[1]	$ (24,838)	[1]	$ (77,521)	[1]	$ (84,224)	[1]

[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.