Components of Net Periodic Pension Cost for Defined Benefit Plan and Defined Contribution Plans (Detail) - USD ($) $ in Thousands	12 Months Ended
	May. 31, 2015	May. 31, 2014	May. 31, 2013
Employee Benefit Plan [Line Items]
Interest cost	$ 1,541	$ 1,403	$ 1,361
Actual return on plan assets	1,846	2,524	4,355
Net amortization and deferral	(3,641)	(4,175)	(5,522)
Net periodic pension cost (benefit) on defined benefit plan	(254)	(248)	194
Defined contribution plans	13,270	12,586	9,955
Total retirement plan cost	$ 13,016	$ 12,338	$ 10,149