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	Feb. 28, 2015	May 31, 2014
Schedule of Available-for-sale Securities [Line Items]
Total investments with unrealized losses, fair value	$ 55,027	$ 35,465
Unrealized losses with a loss position for less than 12 months, fair value	25,927	16,611
Unrealized losses with a loss position for more than 12 months, fair value	29,100	18,854
Total investments with unrealized losses, gross unrealized losses	(3,343)	(1,425)
Unrealized losses with a loss position for less than 12 months, gross unrealized losses	(938)	(845)
Unrealized losses with a loss position for more than 12 months, gross unrealized losses	$ (2,405)	$ (580)