Net Carrying Values of Debt Securities by Contractual Maturity (Detail) (USD $) In Thousands, unless otherwise specified	Feb. 28, 2015	May 31, 2014
Available-for-Sale Securities, amortized cost
Less than one year, amortized cost	$ 3,000
One year through five years, amortized cost	14,221
Six years through ten years, amortized cost	2,955
After ten years, amortized cost	920
Available-for-sale Debt Securities, Amortized Cost Basis, Total	21,096
Available-for-Sale Securities, fair value
Less than one year, fair value	2,998
One year through five years, fair value	14,119
Six years through ten years, fair value	3,021
After ten years, fair value	1,112
Marketable debt securities, carrying value	$ 21,250	$ 23,080