Segment Reporting (Summary of Revenue Attributable to Services Provided by Company) (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
Segment Reporting Information [Line Items]
Revenues	$ 115,002	$ 114,578	$ 356,531	$ 371,637
Collection [Member]
Segment Reporting Information [Line Items]
Revenues	51,459	48,875	157,124	157,265
Disposal [Member]
Segment Reporting Information [Line Items]
Revenues	27,219	30,220	90,569	96,645
Power Generation [Member]
Segment Reporting Information [Line Items]
Revenues	3,400	3,182	8,856	9,415
Processing And Organics [Member]
Segment Reporting Information [Line Items]
Revenues	14,469	12,231	43,378	40,961
Solid Waste Operations [Member]
Segment Reporting Information [Line Items]
Revenues	96,547	94,508	299,927	304,286
Major Accounts [Member]
Segment Reporting Information [Line Items]
Revenues	8,551	9,198	27,296	29,756
Recycling [Member]
Segment Reporting Information [Line Items]
Revenues	$ 9,904	$ 10,872	$ 29,308	$ 37,595