Segment Reporting - Summary of Financial Information by Reportable Segment (Detail) (USD $) In Thousands, unless otherwise specified	3 Months Ended	6 Months Ended
Oct. 31, 2013	Oct. 31, 2012	Oct. 31, 2013	Oct. 31, 2012	Apr. 30, 2013
Segment Reporting Information [Line Items]
Revenue	$ 132,296	$ 116,836	$ 260,854	$ 234,474
Depreciation and amortization	15,669	14,570	30,866	29,279
Operating income (loss)	9,450	4,426	19,188	10,232
Total assets	678,513	669,475	678,513	669,475	663,119
Operating Segments [Member]
Segment Reporting Information [Line Items]
Revenue	132,296	116,836	260,854	234,474
Operating Segments [Member] \| Eastern Region [Member]
Segment Reporting Information [Line Items]
Revenue	40,305	33,047	78,926	66,855
Depreciation and amortization	6,691	6,184	13,072	12,720
Operating income (loss)	2,203	(1,025)	3,829	(1,602)
Total assets	205,181	167,206	205,181	167,206
Operating Segments [Member] \| Western Region [Member]
Segment Reporting Information [Line Items]
Revenue	58,224	54,386	115,571	106,938
Depreciation and amortization	7,311	6,695	14,545	13,117
Operating income (loss)	7,211	6,793	14,107	13,038
Total assets	348,223	355,619	348,223	355,619
Operating Segments [Member] \| Recycling [Member]
Segment Reporting Information [Line Items]
Revenue	11,506	9,810	22,690	20,972
Depreciation and amortization	1,074	1,080	2,139	2,177
Operating income (loss)	(152)	(486)	(195)	(487)
Total assets	49,884	52,462	49,884	52,462
Operating Segments [Member] \| Other [Member]
Segment Reporting Information [Line Items]
Revenue	22,261	19,593	43,667	39,709
Depreciation and amortization	593	611	1,110	1,265
Operating income (loss)	188	(856)	1,447	(717)
Total assets	75,225	94,188	75,225	94,188
Inter-Company Revenue [Member]
Segment Reporting Information [Line Items]
Revenue
Inter-Company Revenue [Member] \| Eastern Region [Member]
Segment Reporting Information [Line Items]
Revenue	10,043	7,868	20,474	16,004
Inter-Company Revenue [Member] \| Western Region [Member]
Segment Reporting Information [Line Items]
Revenue	18,892	17,189	38,153	33,947
Inter-Company Revenue [Member] \| Recycling [Member]
Segment Reporting Information [Line Items]
Revenue	(26)	36	(31)	36
Inter-Company Revenue [Member] \| Other [Member]
Segment Reporting Information [Line Items]
Revenue	491	1,296	1,003	2,511
Inter-Company Revenue [Member] \| Eliminations [Member]
Segment Reporting Information [Line Items]
Revenue	$ (29,400)	$ (26,389)	$ (59,599)	$ (52,498)

Sum of the carrying amounts as of the balance sheet date of all assets that are recognized. Assets are probable future economic benefits obtained or controlled by an entity as a result of past transactions or events.

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

-Publisher FASB

-Name Accounting Standards Codification

-Topic 210

-SubTopic 10

-Section S99

-Paragraph 1

-Subparagraph (SX 210.5-02.18)

-URI http://asc.fasb.org/extlink&oid=6877327&loc=d3e13212-122682

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

-Publisher FASB

-Name Accounting Standards Codification

-Topic 210

-SubTopic 10

-Section S99

-Paragraph 1

-URI http://asc.fasb.org/extlink&oid=6877327&loc=d3e13212-122682

Reference 3: http://www.xbrl.org/2003/role/presentationRef

-Publisher SEC

-Name Regulation S-X (SX)

-Number 210

-Section 03

-Paragraph 12

-Article 7

Reference 4: http://www.xbrl.org/2003/role/presentationRef

-Publisher SEC

-Name Regulation S-X (SX)

-Number 210

-Section 02

-Paragraph 18

-Article 5

+ Details

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

- Definition

The aggregate expense recognized in the current period that allocates the cost of tangible assets, intangible assets, or depleting assets to periods that benefit from use of the assets.

+ References

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

-Publisher FASB

-Name Accounting Standards Codification

-Topic 230

-SubTopic 10

-Section 45

-Paragraph 28

-Subparagraph (b)

-URI http://asc.fasb.org/extlink&oid=31042434&loc=d3e3602-108585

+ Details

Name:	us-gaap_DepreciationDepletionAndAmortization
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	duration

- Definition

The net result for the period of deducting operating expenses from operating revenues.

+ References

No definition available.

+ Details

Name:	us-gaap_OperatingIncomeLoss
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	duration

- Definition

Amount of revenue recognized from goods sold, services rendered, insurance premiums, or other activities that constitute an earning process. Includes, but is not limited to, investment and interest income before deduction of interest expense when recognized as a component of revenue, and sales and trading gain (loss).

+ References

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

-Publisher FASB

-Name Accounting Standards Codification

-Topic 225

-SubTopic 10

-Section S99

-Paragraph 2

-Subparagraph (SX 210.5-03.1)

-URI http://asc.fasb.org/extlink&oid=26872669&loc=d3e20235-122688

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

-Publisher SEC

-Name Regulation S-X (SX)

-Number 210

-Section 03

-Paragraph 1

-Article 5

+ Details

Name:	us-gaap_Revenues
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	duration

- Details

Name:	us-gaap_SegmentReportingInformationLineItems
Namespace Prefix:	us-gaap_
Data Type:	xbrli:stringItemType
Balance Type:	na
Period Type:	duration