Business Acquisitions (Narrative) (Detail) (USD $) In Thousands, unless otherwise specified	3 Months Ended		9 Months Ended									9 Months Ended	2 Months Ended
	Jan. 31, 2013	Jan. 31, 2012	Jan. 31, 2013 Property	Jan. 31, 2012 Property	Jan. 31, 2013 Minimum [Member] Y	Jan. 31, 2012 Minimum [Member] Y	Jan. 31, 2013 Maximum [Member] Y	Jan. 31, 2012 Maximum [Member] Y	Jan. 31, 2013 Business Acquisition [Member]	Jan. 31, 2012 Business Acquisition [Member]	Jan. 31, 2013 Bestway Disposal Services And Bbi Waste Services [Member]	Jan. 31, 2013 Bestway Disposal Services And Bbi Waste Services [Member] Class A Common Stock [Member]	Jan. 31, 2013 BBI Acquisition [Member]
Business Acquisition [Line Items]
Total consideration	$ 5,384	$ 2,284	$ 5,384	$ 2,284							$ 22,650
Business acquisition, cash	4,854	2,102	4,854	2,102							20,000
Business acquisition, hold backs to sellers	180	182	180	182					530	182
Business acquisition, shares												625
Business acquisition, stock value	2,650		2,650								2,650
Goodwill not expected to be deductible for tax purpose											8,078
Number of business acquired			4	5
Acquired intangible assets, amortization period					5	5	10	10
Deferred tax liability recorded through goodwill													5,242
Revenues	$ 115,002	$ 114,578	$ 356,531	$ 371,637									$ 2,924