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Segment Reporting - Summary of Revenue Attributable to Services Provided by Company (Detail) (USD $) In Thousands, unless otherwise specified	3 Months Ended												12 Months Ended
	Apr. 30, 2013	Jan. 31, 2013	Oct. 31, 2012	Jul. 31, 2012	Apr. 30, 2012	Jan. 31, 2012	Oct. 31, 2011	Jul. 31, 2011	Apr. 30, 2011	Jan. 31, 2011	Oct. 31, 2010	Jul. 31, 2010	Apr. 30, 2013	Apr. 30, 2012	Apr. 30, 2011
Segment Reporting Information [Line Items]
Revenues	$ 108,694	$ 112,167	$ 116,836	$ 117,638	$ 106,375	$ 111,520	$ 125,989	$ 124,066	$ 106,772	$ 109,076	$ 119,694	$ 119,143	$ 455,335	$ 467,950	$ 454,685
Revenues percentage	100.00%				100.00%				100.00%				100.00%	100.00%	100.00%
Collection [Member]
Segment Reporting Information [Line Items]
Revenues													208,973	205,296	199,892
Revenues percentage	45.90%				43.90%				44.00%				45.90%	43.90%	44.00%
Disposal [Member]
Segment Reporting Information [Line Items]
Revenues													115,049	123,620	118,831
Revenues percentage	25.30%				26.40%				26.10%				25.30%	26.40%	26.10%
Power generation [Member]
Segment Reporting Information [Line Items]
Revenues													11,354	11,894	12,831
Revenues percentage	2.40%				2.60%				2.80%				2.40%	2.60%	2.80%
Organics and processing [Member]
Segment Reporting Information [Line Items]
Revenues													45,373	40,904	39,211
Revenues percentage	10.00%				8.70%				8.60%				10.00%	8.70%	8.60%
Solid waste operations [Member]
Segment Reporting Information [Line Items]
Revenues													380,749	381,714	370,765
Revenues percentage	83.60%				81.60%				81.50%				83.60%	81.60%	81.50%
Customer Resource Solutions [Member]
Segment Reporting Information [Line Items]
Revenues													35,455	38,302	40,363
Revenues percentage	7.80%				8.20%				8.90%				7.80%	8.20%	8.90%
Recycling [Member]
Segment Reporting Information [Line Items]
Revenues													$ 39,131	$ 47,934	$ 43,557
Revenues percentage	8.60%				10.20%				9.60%				8.60%	10.20%	9.60%

X

- Definition

Aggregate revenue recognized during the period (derived from goods sold, services rendered, insurance premiums, or other activities that constitute an entity's earning process). For financial services companies, also includes investment and interest income, and sales and trading gains.

+ 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=6880815&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

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X

- Definition

The percentage of net product revenue to total net revenue from the sale of goods as of year end.

+ References

No definition available.

+ Details

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X

- Details

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