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	Feb. 28, 2019	May 31, 2018
Investments Debt And Equity Securities [Abstract]
Total investments with unrealized losses, fair value	$ 16,582	$ 106,253
Unrealized losses with a loss position for less than 12 months, fair value	822	68,376
Unrealized losses with a loss position for more than 12 months, fair value	15,760	37,877
Total investments with unrealized losses, gross unrealized losses	(383)	(4,527)
Unrealized losses with a loss position for less than 12 months, gross unrealized losses	(4)	(1,570)
Unrealized losses with a loss position for more than 12 months, gross unrealized losses	$ (379)	$ (2,957)