Components of Net Periodic Pension Cost for the Defined Benefit Plan and Defined Contribution Plans (Detail) - USD ($) $ in Thousands	12 Months Ended
	May 31, 2017	May 31, 2016	May 31, 2015
Defined Benefit Plan Disclosure [Line Items]
Interest cost	$ 1,527	$ 1,621	$ 1,541
Actuarial (return) loss on plan assets	(2,224)	1,154	(1,846)
Net amortization and deferral	1,025	(2,664)	51
Net periodic pension cost (benefit) on defined benefit plan	328	111	(254)
Defined contribution plans	14,542	13,300	13,270
Total retirement plan cost	$ 14,870	$ 13,411	$ 13,016