Summary of Securities in Unrealized Loss Position and Included in Accumulated Other Comprehensive Income (Loss), Aggregated by Length of Time Investments (Detail) - USD ($) $ in Thousands	Nov. 30, 2015	May. 31, 2015
Investments Debt And Equity Securities [Abstract]
Total investments with unrealized losses, fair value	$ 97,059	$ 58,978
Unrealized losses with a loss position for less than 12 months, fair value	55,461	32,693
Unrealized losses with a loss position for more than 12 months, fair value	41,598	26,285
Total investments with unrealized losses, gross unrealized losses	(7,889)	(3,815)
Unrealized losses with a loss position for less than 12 months, gross unrealized losses	(4,872)	(1,441)
Unrealized losses with a loss position for more than 12 months, gross unrealized losses	$ (3,017)	$ (2,374)