Summary of Securities in Unrealized Loss Position and Included in Accumulated Other Comprehensive Income, Aggregated by Length of Time Investments (Detail) (USD $) In Thousands, unless otherwise specified	Aug. 31, 2014	May 31, 2014
Schedule of Available-for-sale Securities [Line Items]
Total investments with unrealized losses, fair value	$ 41,958	$ 35,465
Unrealized losses with a loss position for less than 12 months, fair value	26,504	16,611
Unrealized losses with a loss position for more than 12 months, fair value	15,454	18,854
Total investments with unrealized losses, gross unrealized losses	(1,411)	(1,425)
Unrealized losses with a loss position for less than 12 months, gross unrealized losses	(952)	(845)
Unrealized losses with a loss position for more than 12 months, gross unrealized losses	$ (459)	$ (580)