Defined Benefit Pension Plan and Postretirement Health Benefit Plan Expense (Detail) (Riverside Defined Benefit Plans, USD $) In Thousands, unless otherwise specified	3 Months Ended		9 Months Ended
	Feb. 28, 2013	Feb. 29, 2012	Feb. 28, 2013	Feb. 29, 2012
Pension Plans, Defined Benefit
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Service cost	$ 61	$ 135	$ 277	$ 403
Interest cost	635	760	1,978	2,280
Expected return on plan assets	770	777	2,288	2,331
Amortization of net actuarial loss	(217)	(430)	(1,165)	(1,291)
Defined benefit pension plan, total	143	548	1,132	1,643
Defined Benefit Postretirement Health Coverage
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Service cost	27	23	81	73
Interest cost	88	104	264	312
Amortization of prior service cost	(194)	(193)	(582)	(581)
Amortization of net actuarial loss	(129)	(142)	(387)	(424)
Defined benefit pension plan, total	$ 50	$ 76	$ 150	$ 228