Segment Reporting - Summary of Financial Information by Reportable Segment (Detail) (USD $) In Thousands, unless otherwise specified	3 Months Ended	9 Months Ended
Jan. 31, 2014	Jan. 31, 2013	Jan. 31, 2014	Jan. 31, 2013	Apr. 30, 2013
Segment Reporting Information [Line Items]
Revenue	$ 117,852	$ 112,167	$ 378,706	$ 346,641
Depreciation and amortization	14,396	13,964	45,261	43,244
Operating income (loss)	(1,298)	78	17,889	10,310
Total assets	669,442	673,153	669,442	673,153	663,119
Operating Segments [Member]
Segment Reporting Information [Line Items]
Revenue	117,852	112,167	378,706	346,641
Operating Segments [Member] \| Eastern Region [Member]
Segment Reporting Information [Line Items]
Revenue	35,132	32,311	114,059	99,165
Depreciation and amortization	5,875	5,791	18,947	18,512
Operating income (loss)	(3,122)	(2,783)	706	(4,385)
Total assets	204,330	199,251	204,330	199,251
Operating Segments [Member] \| Western Region [Member]
Segment Reporting Information [Line Items]
Revenue	50,745	50,274	166,315	157,212
Depreciation and amortization	6,828	6,590	21,373	19,707
Operating income (loss)	3,315	3,937	17,422	16,975
Total assets	345,677	353,418	345,677	353,418
Operating Segments [Member] \| Recycling [Member]
Segment Reporting Information [Line Items]
Revenue	10,160	10,702	32,850	31,674
Depreciation and amortization	1,056	1,067	3,195	3,244
Operating income (loss)	(1,529)	(120)	(1,724)	(606)
Total assets	48,859	51,814	48,859	51,814
Operating Segments [Member] \| Other [Member]
Segment Reporting Information [Line Items]
Revenue	21,815	18,880	65,482	58,590
Depreciation and amortization	637	516	1,746	1,781
Operating income (loss)	38	(956)	1,485	(1,674)
Total assets	70,576	68,670	70,576	68,670
Inter-Company Revenue [Member]
Segment Reporting Information [Line Items]
Revenue
Inter-Company Revenue [Member] \| Eastern Region [Member]
Segment Reporting Information [Line Items]
Revenue	9,043	7,158	29,517	23,162
Inter-Company Revenue [Member] \| Western Region [Member]
Segment Reporting Information [Line Items]
Revenue	16,829	15,954	54,983	49,901
Inter-Company Revenue [Member] \| Recycling [Member]
Segment Reporting Information [Line Items]
Revenue	(44)	41	(76)	77
Inter-Company Revenue [Member] \| Other [Member]
Segment Reporting Information [Line Items]
Revenue	463	736	1,466	3,247
Inter-Company Revenue [Member] \| Eliminations [Member]
Segment Reporting Information [Line Items]
Revenue	$ (26,291)	$ (23,889)	$ (85,890)	$ (76,387)

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

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- 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

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

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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

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-Name Regulation S-X (SX)

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