Defined Benefit Pension Plan and Postretirement Health Benefit Plan Expense (Detail) (Riverside Defined Benefit Plans, USD $) In Thousands, unless otherwise specified	3 Months Ended
	Aug. 31, 2013	Aug. 31, 2012
Pension Plans, Defined Benefit
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Service cost	$ 0	$ 155
Interest cost	683	708
Expected return on plan assets	(851)	(748)
Amortization of net actuarial loss	154	732
Defined benefit pension plan, total	(14)	847
Defined Benefit Postretirement Health Coverage
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Service cost	25	27
Interest cost	72	88
Amortization of prior service cost	(194)	(194)
Amortization of net actuarial loss	55	129
Defined benefit pension plan, total	$ (42)	$ 50