Summary of Significant Accounting Policies and Practices - Additional Information (Detail) (USD $)	3 Months Ended		9 Months Ended
	Feb. 28, 2013	Feb. 29, 2012	Feb. 28, 2013	Feb. 29, 2012	May 31, 2012
Significant Accounting Policies [Line Items]
Goodwill	$ 104,787,000		$ 104,787,000		$ 95,002,000
Deferred tax assets recovery from future taxable income percentage			50.00%
Workers compensation our self-insured retention			1,000,000
Automobile liability self-insured retention			500,000
General liability claims we have an effective self-insured retention			3,000,000
Medical claims, our self-insured retention			150,000
Environmental liability claims, our self-insured retention			500,000
Amount of earned but unbilled revenue included in accounts receivable	$ 20,569,000		$ 20,569,000		$ 20,561,000
Single customer accounts consolidated revenue			10.00%
Options to purchase shares of common stock outstanding	0	578,000	1,000	643,000