Summary of Securities in Unrealized Loss Position and Included in Accumulated Other Comprehensive Income, Aggregated by Length of Time Investments had been in Unrealized Loss Position (Detail) (USD $) In Thousands, unless otherwise specified	Feb. 29, 2012	May 31, 2011
Schedule of Available-for-sale Securities [Line Items]
Total investments with unrealized losses, fair value	$ 42,663	$ 21,612
Unrealized losses with a loss position for less than 12 months, fair value	33,893	19,721
Unrealized losses with a loss position for more than 12 months, fair value	8,770	1,891
Total investments with unrealized losses, gross unrealized losses	(1,388)	(1,518)
Unrealized losses with a loss position for less than 12 months, gross unrealized losses	(1,139)	(1,301)
Unrealized losses with a loss position for more than 12 months, gross unrealized losses	$ (249)	$ (217)