Segment Reporting - Summary of Revenue Attributable to Services Provided by Company (Detail) (USD $) In Thousands, unless otherwise specified	3 Months Ended		6 Months Ended
	Oct. 31, 2013	Oct. 31, 2012	Oct. 31, 2013	Oct. 31, 2012
Segment Reporting Information [Line Items]
Revenues	$ 132,296	$ 116,836	$ 260,854	$ 234,474
Operating Segments [Member]
Segment Reporting Information [Line Items]
Revenues	132,296	116,836	260,854	234,474
Operating Segments [Member] | Solid Waste Operations [Member]
Segment Reporting Information [Line Items]
Revenues	100,798	89,411	199,128	177,509
Operating Segments [Member] | Solid Waste Operations [Member] | Reportable Subsegments [Member] | Collection [Member]
Segment Reporting Information [Line Items]
Revenues	58,932	52,632	117,245	105,665
Operating Segments [Member] | Solid Waste Operations [Member] | Reportable Subsegments [Member] | Disposal [Member]
Segment Reporting Information [Line Items]
Revenues	37,374	32,382	72,497	63,349
Operating Segments [Member] | Solid Waste Operations [Member] | Reportable Subsegments [Member] | Power Generation [Member]
Segment Reporting Information [Line Items]
Revenues	1,980	2,793	4,022	5,456
Operating Segments [Member] | Solid Waste Operations [Member] | Reportable Subsegments [Member] | Processing [Member]
Segment Reporting Information [Line Items]
Revenues	2,512	1,604	5,364	3,039
Operating Segments [Member] | Organics [Member]
Segment Reporting Information [Line Items]
Revenues	9,474	8,394	19,350	17,247
Operating Segments [Member] | Customer Solutions [Member]
Segment Reporting Information [Line Items]
Revenues	10,518	9,221	19,686	18,746
Operating Segments [Member] | Recycling [Member]
Segment Reporting Information [Line Items]
Revenues	$ 11,506	$ 9,810	$ 22,690	$ 20,972