Retirement Plans - Amount of Financial Security Plan Benefit Expense Charged to Costs and Expenses (Details) (Financial Security Defined Benefit Plans, USD $) In Thousands, unless otherwise specified	12 Months Ended
	May 31, 2013	May 31, 2012	May 31, 2011
Financial Security Defined Benefit Plans
Deferred Compensation Arrangement with Individual, Postretirement Benefits [Line Items]
Service cost	$ 2,370	$ 2,147	$ 2,213
Interest cost	2,364	2,517	2,311
Amortization of net actuarial loss	(38)	4,366	2,347
Net periodic benefit cost	$ 4,696	$ 9,030	$ 6,871