Net Carrying Values of Debt Securities by Contractual Maturity (Detail) (USD $) In Thousands, unless otherwise specified	Aug. 31, 2013	May 31, 2013
Available-for-Sale Securities, amortized cost
Less than one year, amortized cost	$ 3,081
One year through five years, amortized cost	13,043
Six years through ten years, amortized cost	4,439
After ten years, amortized cost	1,662
Available-for-sale Securities, Debt Maturities, Amortized Cost Basis, Total	22,225
Available-for-Sale Securities, fair value
Less than one year, fair value	3,093
One year through five years, fair value	13,100
Six years through ten years, fair value	4,322
After ten years, fair value	1,787
Marketable debt securities, carrying value	$ 22,302	$ 22,801