Goodwill and Intangible Assets - Schedule of Activity and Balances Related to Goodwill (Detail) (USD $) In Thousands, unless otherwise specified	6 Months Ended				6 Months Ended
Oct. 31, 2014	Oct. 31, 2013	Oct. 31, 2014 Eastern Region [Member]	Apr. 30, 2014 Eastern Region [Member]	Oct. 31, 2014 Western Region [Member]	Oct. 31, 2014 Recycling [Member]	Apr. 30, 2014 Recycling [Member]	Oct. 31, 2014 Other [Member]	Apr. 30, 2014 Other [Member]
Goodwill And Intangible Assets [Line Items]
Goodwill, beginning balance	$ 119,139	$ 2,272	$ 17,429	$ 17,429	$ 87,666	$ 12,315	$ 12,315	$ 1,729	$ 1,729
Acquisitions	31				31
Goodwill, ending balance	$ 119,170	$ 2,272	$ 17,429	$ 17,429	$ 87,697	$ 12,315	$ 12,315	$ 1,729	$ 1,729

Amount after accumulated impairment loss of an asset representing future economic benefits arising from other assets acquired in a business combination that are not individually identified and separately recognized.

Reference 1: http://www.xbrl.org/2003/role/presentationRef

-Publisher FASB

-Name Accounting Standards Codification

-Topic 350

-SubTopic 20

-Section 50

-Paragraph 1

-URI http://asc.fasb.org/extlink&oid=35741047&loc=d3e13816-109267

Reference 2: http://www.xbrl.org/2003/role/presentationRef

-Publisher FASB

-Name Accounting Standards Codification

-Topic 350

-SubTopic 20

-Section 45

-Paragraph 1

-URI http://asc.fasb.org/extlink&oid=6388280&loc=d3e13770-109266

Name:	us-gaap_Goodwill
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	instant