Segment Reporting - Summary of Financial Information by Reportable Segment (Detail) (USD $) In Thousands, unless otherwise specified	3 Months Ended
	Jul. 31, 2014	Jul. 31, 2013	Apr. 30, 2014
Segment Reporting Information [Line Items]
Revenue	$ 141,387	$ 128,558
Depreciation and amortization	16,368	15,197
Operating income (loss)	9,338	9,737
Total assets	656,602	674,744	649,897
Operating Segments [Member]
Segment Reporting Information [Line Items]
Revenue	141,387	128,558
Operating Segments [Member] | Recycling [Member]
Segment Reporting Information [Line Items]
Revenue	12,472	11,184
Depreciation and amortization	1,065	1,065
Operating income (loss)	(111)	(44)
Total assets	49,968	50,857
Operating Segments [Member] | Other [Member]
Segment Reporting Information [Line Items]
Revenue	26,534	21,406
Depreciation and amortization	596	518
Operating income (loss)	409	1,259
Total assets	70,560	71,491
Operating Segments [Member] | Eastern [Member]
Segment Reporting Information [Line Items]
Revenue	41,205	38,621
Depreciation and amortization	6,736	6,381
Operating income (loss)	1,303	1,626
Total assets	204,569	205,410
Operating Segments [Member] | Western [Member]
Segment Reporting Information [Line Items]
Revenue	61,176	57,347
Depreciation and amortization	7,971	7,233
Operating income (loss)	7,737	6,896
Total assets	331,505	346,986
Inter-Company Revenue [Member] | Recycling [Member]
Segment Reporting Information [Line Items]
Revenue	(54)	(5)
Inter-Company Revenue [Member] | Other [Member]
Segment Reporting Information [Line Items]
Revenue	637	512
Inter-Company Revenue [Member] | Eliminations [Member]
Segment Reporting Information [Line Items]
Revenue	(32,397)	(30,199)
Inter-Company Revenue [Member] | Eastern [Member]
Segment Reporting Information [Line Items]
Revenue	12,236	10,431
Inter-Company Revenue [Member] | Western [Member]
Segment Reporting Information [Line Items]
Revenue	$ 19,578	$ 19,261