Goodwill and Intangible Assets - Schedule of Activity and Balances Related to Goodwill (Detail) (USD $) In Thousands, unless otherwise specified	6 Months Ended
	Oct. 31, 2013
Goodwill And Intangible Assets [Line Items]
Goodwill, beginning balance	$ 115,928
Acquisitions	2,329
Goodwill, ending balance	118,257
Eastern Region [Member]
Goodwill And Intangible Assets [Line Items]
Goodwill, beginning balance	16,858
Acquisitions	133
Goodwill, ending balance	16,991
Western Region [Member]
Goodwill And Intangible Assets [Line Items]
Goodwill, beginning balance	86,880
Acquisitions	467
Goodwill, ending balance	87,347
Recycling [Member]
Goodwill And Intangible Assets [Line Items]
Goodwill, beginning balance	12,190
Acquisitions
Goodwill, ending balance	12,190
Other [Member]
Goodwill And Intangible Assets [Line Items]
Goodwill, beginning balance
Acquisitions	1,729
Goodwill, ending balance	$ 1,729