Retirement Plans (Schedule Of Defined Benefit Pension Plan And Postretirement Health Benefit Plan Expense) (Details) (USD $) In Thousands, unless otherwise specified	3 Months Ended		6 Months Ended
	Nov. 30, 2011	Nov. 30, 2010	Nov. 30, 2011	Nov. 30, 2010
Approximate number of employees and retirees of subsidiary			600
Defined Benefit Pension Plan [Member]
Service cost	$ 134	$ 126	$ 268	$ 251
Interest cost	760	737	1,520	1,475
Expected return on plan assets	(777)	(685)	(1,554)	(1,370)
Amortization of net actuarial loss	430	517	861	1,034
Defined benefit pension plan, total	547	695	1,095	1,390
Postretirement Health Benefit Plan [Member]
Service cost	25	29	50	57
Interest cost	104	106	208	212
Amortization of prior service cost	194	193	388	387
Amortization of net actuarial loss	141	153	282	307
Defined benefit pension plan, total	$ 76	$ 95	$ 152	$ 189