Net Carrying Values of Debt Securities by Contractual Maturity (Detail) - USD ($) $ in Thousands	Aug. 31, 2015	May. 31, 2015
Available-for-Sale Securities, amortized cost
Less than one year, amortized cost	$ 3,609
One year through five years, amortized cost	15,787
Six years through ten years, amortized cost	1,919
After ten years, amortized cost	1,181
Available-for-sale Debt Securities, Amortized Cost Basis, Total	22,496
Available-for-Sale Securities, fair value
Less than one year, fair value	3,609
One year through five years, fair value	15,724
Six years through ten years, fair value	1,937
After ten years, fair value	1,336
Marketable debt securities, carrying value	$ 22,606	$ 22,904