Summary of Available-for-Sale Securities in Unrealized Loss Position and Included in Accumulated Other Comprehensive (Loss), Aggregated by Length of Time Investments (Detail) - USD ($) $ in Thousands	Nov. 30, 2018	May 31, 2018
Investments Debt And Equity Securities [Abstract]
Total investments with unrealized losses, fair value	$ 20,215	$ 106,253
Unrealized losses with a loss position for less than 12 months, fair value	2,646	68,376
Unrealized losses with a loss position for more than 12 months, fair value	17,569	37,877
Total investments with unrealized losses, gross unrealized losses	(535)	(4,527)
Unrealized losses with a loss position for less than 12 months, gross unrealized losses	(19)	(1,570)
Unrealized losses with a loss position for more than 12 months, gross unrealized losses	$ (516)	$ (2,957)