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, 2013	May 31, 2012
Schedule of Available-for-sale Securities [Line Items]
Total investments with unrealized losses, fair value	$ 45,700	$ 43,772
Unrealized losses with a loss position for less than 12 months, fair value	44,848	42,114
Unrealized losses with a loss position for more than 12 months, fair value	852	1,658
Total investments with unrealized losses, gross unrealized losses	(3,562)	(3,078)
Unrealized losses with a loss position for less than 12 months, gross unrealized losses	(3,357)	(2,596)
Unrealized losses with a loss position for more than 12 months, gross unrealized losses	$ (205)	$ (482)