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	Aug. 31, 2011	May 31, 2011
Schedule of Trading Securities and Other Trading Assets [Line Items]
Total investments with unrealized losses, fair value	$ 35,734	$ 21,612
Unrealized losses with a loss position for less than 12 months, fair value	35,682	19,721
Unrealized losses with a loss position for more than 12 months, fair value	52	1,891
Total investments with unrealized losses, gross unrealized losses	(3,910)	(1,518)
Unrealized losses with a loss position for less than 12 months, gross unrealized losses	(3,881)	(1,301)
Unrealized losses with a loss position for more than 12 months, gross unrealized losses	$ (29)	$ (217)