Defined Benefit Pension Plan and Postretirement Health Benefit Plan Expense (Detail) (Riverside Defined Benefit Plans, USD $) In Thousands, unless otherwise specified	12 Months Ended
	May 31, 2012	May 31, 2011	May 31, 2010
Weighted average assumptions used to determine net cost
Assumed long-term rate of return on pension plan assets	7.60%
Pension Plans, Defined Benefit
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Service cost	$ 537	$ 502	$ 457
Interest cost	3,040	2,951	3,166
Expected return on plan assets	(3,108)	(2,740)	(2,342)
Amortization of net actuarial loss	1,722	2,068	2,000
Defined benefit pension plan, total	2,191	2,781	3,281
Weighted average assumptions used to determine net cost
Assumed discount rate	5.35%	5.60%	6.80%
Assumed long-term rate of return on pension plan assets	7.60%	8.25%	8.25%
Average long-term pay progression	3.00%	3.00%	3.00%
Defined Benefit Postretirement Health Coverage
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Service cost	98	114	106
Interest cost	415	425	466
Amortization of prior service credit	(775)	(775)	(774)
Amortization of net actuarial loss	566	614	603
Defined benefit pension plan, total	$ 304	$ 378	$ 401
Weighted average assumptions used to determine net cost
Assumed discount rate	5.35%	5.60%	6.80%