Retirement Plans - Reconciliation of Financial Security Plan Benefit Obligation (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]
Benefit obligation at beginning of year	$ 54,230	$ 48,091
Service cost	2,370	2,147	2,213
Interest cost	2,364	2,517	2,311
Actuarial loss (gain)	(38)	4,366
Benefits paid	(3,323)	(2,891)
Benefit obligation at end of year	55,603	54,230	48,091
Funded status at end of year	$ (55,603)	$ (54,230)