Retirement Plans - Defined Benefit Pension Plan and Postretirement Health Benefit Plan Expense (Details) (Riverside Defined Benefit Plans, USD $) In Thousands, unless otherwise specified	12 Months Ended
	May 31, 2013	May 31, 2012	May 31, 2011
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Assumed long-term rate of return	7.30%
Pension Plans
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Service cost	$ 339	$ 537	$ 502
Interest cost	2,613	3,040	2,951
Expected return on plan assets	(3,059)	(3,108)	(2,740)
Amortization of net actuarial loss	1,381	1,722	2,068
Net periodic benefit cost	1,274	2,191	2,781
Assumed discount rate		5.35%	5.60%
Assumed long-term rate of return	7.30%	7.60%	8.25%
Average long-term pay progression	3.00%	3.00%	3.00%
Health Benefit
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Service cost	106	98	114
Interest cost	352	415	425
Amortization of prior service credit	(775)	(775)	(775)
Amortization of net actuarial loss	515	566	614
Net periodic benefit cost	$ 198	$ 304	$ 378
Assumed discount rate	4.35%	5.35%	5.60%
Maximum | Pension Plans
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Assumed discount rate	4.35%
Minimum | Pension Plans
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Assumed discount rate	3.90%