Summary of Securities that were in an Unrealized Loss Position and that were Included in Accumulated Other Comprehensive Income, Aggregated by the Length of Time the Investments had been in that Position (Detail) (USD $) In Thousands, unless otherwise specified	Nov. 30, 2011	May 31, 2011
Schedule of Available-for-sale Securities [Line Items]
Total investments with unrealized losses, fair value	$ 40,559	$ 21,612
Unrealized losses with a loss position for less than 12 months, fair value	40,508	19,721
Unrealized losses with a loss position for more than 12 months, fair value	51	1,891
Total investments with unrealized losses, gross unrealized losses	(2,743)	(1,518)
Unrealized losses with a loss position for less than 12 months, gross unrealized losses	(2,713)	(1,301)
Unrealized losses with a loss position for more than 12 months, gross unrealized losses	$ (30)	$ (217)