Defined Benefit Pension Plan and Postretirement Health Benefit Plan Expense (Detail) (Riverside Defined Benefit Plans, USD $) In Thousands, unless otherwise specified	3 Months Ended		6 Months Ended
	Nov. 30, 2013	Nov. 30, 2012	Nov. 30, 2013	Nov. 30, 2012
Pension Plans, Defined Benefit
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Service cost	$ 0	$ 61	$ 0	$ 216
Interest cost	683	635	1,366	1,343
Expected return on plan assets	(851)	(770)	(1,702)	(1,518)
Amortization of net actuarial loss	154	217	308	948
Defined benefit pension plan, total	(14)	143	(28)	989
Defined Benefit Postretirement Health Coverage
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Service cost	25	27	50	54
Interest cost	72	88	144	176
Amortization of prior service cost	(194)	(194)	(388)	(388)
Amortization of net actuarial loss	57	129	112	258
Defined benefit pension plan, total	$ (40)	$ 50	$ (82)	$ 100