Segment Reporting - Summary of Revenue Attributable to Services Provided by Company (Detail) (USD $) In Thousands, unless otherwise specified	3 Months Ended
	Jul. 31, 2014	Jul. 31, 2013
Segment Reporting Information [Line Items]
Revenues	$ 141,387	$ 128,558
Operating Segments [Member]
Segment Reporting Information [Line Items]
Revenues	141,387	128,558
Operating Segments [Member] | Solid Waste Operations [Member]
Segment Reporting Information [Line Items]
Revenues	104,902	98,328
Operating Segments [Member] | Solid Waste Operations [Member] | Reportable Subsegments [Member] | Collection [Member]
Segment Reporting Information [Line Items]
Revenues	60,011	58,213
Operating Segments [Member] | Solid Waste Operations [Member] | Reportable Subsegments [Member] | Disposal [Member]
Segment Reporting Information [Line Items]
Revenues	40,092	35,123
Operating Segments [Member] | Solid Waste Operations [Member] | Reportable Subsegments [Member] | Power Generation [Member]
Segment Reporting Information [Line Items]
Revenues	1,940	2,041
Operating Segments [Member] | Solid Waste Operations [Member] | Reportable Subsegments [Member] | Processing [Member]
Segment Reporting Information [Line Items]
Revenues	2,859	2,951
Operating Segments [Member] | Organics [Member]
Segment Reporting Information [Line Items]
Revenues	10,662	9,877
Operating Segments [Member] | Customer Solutions [Member]
Segment Reporting Information [Line Items]
Revenues	13,351	9,169
Operating Segments [Member] | Recycling [Member]
Segment Reporting Information [Line Items]
Revenues	$ 12,472	$ 11,184