Defined Contribution Benefit Plan - Additional Information (Detail) - USD ($)	12 Months Ended
	May. 31, 2015	May. 31, 2014	May. 31, 2013
Defined Benefit Plan Disclosure [Line Items]
Defined contribution 401(K) benefit expense	$ 1,051,000	$ 954,000	$ 863,000
First 3% employees deferred amount
Defined Benefit Plan Disclosure [Line Items]
Defined contribution plan employer matching contribution	100.00%
Next 2% employees deferred amount
Defined Benefit Plan Disclosure [Line Items]
Defined contribution plan employer matching contribution	50.00%