Segment Reporting - Summary of Financial Information by Reportable Segment (Detail) (USD $) In Thousands, unless otherwise specified	3 Months Ended		6 Months Ended
	Oct. 31, 2014	Oct. 31, 2013	Oct. 31, 2014	Oct. 31, 2013	Apr. 30, 2014
Segment Reporting Information [Line Items]
Revenue	$ 141,341	$ 132,296	$ 282,728	$ 260,854
Depreciation and amortization	16,028	15,669	32,396	30,866
Operating income (loss)	10,064	9,450	19,403	19,188
Total assets	661,794	678,513	661,794	678,513	649,897
Operating Segments [Member]
Segment Reporting Information [Line Items]
Revenue	141,341	132,296	282,728	260,854
Operating Segments [Member] | Recycling [Member]
Segment Reporting Information [Line Items]
Revenue	13,261	11,506	25,733	22,690
Depreciation and amortization	1,081	1,074	2,146	2,139
Operating income (loss)	380	(152)	269	(195)
Total assets	52,218	49,884	52,218	49,884
Operating Segments [Member] | Other [Member]
Segment Reporting Information [Line Items]
Revenue	26,014	22,261	52,548	43,667
Depreciation and amortization	588	593	1,184	1,110
Operating income (loss)	162	188	571	1,447
Total assets	69,968	75,225	69,968	75,225
Operating Segments [Member] | Eastern Region [Member]
Segment Reporting Information [Line Items]
Revenue	42,052	40,305	83,257	78,926
Depreciation and amortization	7,014	6,691	13,750	13,072
Operating income (loss)	2,200	2,203	3,503	3,829
Total assets	206,860	205,181	206,860	205,181
Operating Segments [Member] | Western Region [Member]
Segment Reporting Information [Line Items]
Revenue	60,014	58,224	121,190	115,571
Depreciation and amortization	7,345	7,311	15,316	14,545
Operating income (loss)	7,322	7,211	15,060	14,107
Total assets	332,748	348,223	332,748	348,223
Inter-Company Revenue [Member]
Segment Reporting Information [Line Items]
Revenue	(32,782)	(29,400)	(65,179)	(59,599)
Inter-Company Revenue [Member] | Recycling [Member]
Segment Reporting Information [Line Items]
Revenue	(50)	(26)	(104)	(31)
Inter-Company Revenue [Member] | Other [Member]
Segment Reporting Information [Line Items]
Revenue	667	491	1,304	1,003
Inter-Company Revenue [Member] | Eastern Region [Member]
Segment Reporting Information [Line Items]
Revenue	12,541	10,043	24,777	20,474
Inter-Company Revenue [Member] | Western Region [Member]
Segment Reporting Information [Line Items]
Revenue	$ 19,624	$ 18,892	$ 39,202	$ 38,153