Business Acquisitions - Additional Information (Detail) (USD $) In Thousands, unless otherwise specified	6 Months Ended		3 Months Ended						6 Months Ended
	Oct. 31, 2013	Oct. 31, 2012	Oct. 31, 2013 Minimum [Member]	Oct. 31, 2012 Minimum [Member]	Oct. 31, 2013 Maximum [Member]	Oct. 31, 2012 Maximum [Member]	Oct. 31, 2013 Industrial Environmental Service Management Business [Member]	Oct. 31, 2013 Industrial Environmental Service Management Business [Member] Maximum [Member]	Oct. 31, 2013 Western Region [Member] Property	Oct. 31, 2012 Western Region [Member] Property	Oct. 31, 2013 Eastern Region [Member] Property	Oct. 31, 2013 Other Region [Member]
Business Acquisition [Line Items]
Number of businesses acquired									2	2	1
Total consideration									$ 1,891	$ 4,761	$ 209	$ 2,494
Business acquisition, cash	2,793	4,635							1,473	4,285		1,320
Business acquisition, advanced customer billings	245								36
Business acquisition, hold backs to sellers									382	476		330
Business combination consideration transferred contingent consideration							844
Business combination consideration transferred contingent consideration, upper limit								$ 1,000
Acquired intangible assets, amortization period			5 years	5 years	10 years	10 years