Summary of Securities in Unrealized Loss Position and Included in Accumulated Other Comprehensive (Loss), Aggregated by Length of Time Investments (Detail) - USD ($) $ in Thousands	Aug. 31, 2018	May 31, 2018
Investments Debt And Equity Securities [Abstract]
Total investments with unrealized losses, fair value	$ 87,434	$ 106,253
Unrealized losses with a loss position for less than 12 months, fair value	50,465	68,376
Unrealized losses with a loss position for more than 12 months, fair value	36,969	37,877
Total investments with unrealized losses, gross unrealized losses	(4,975)	(4,527)
Unrealized losses with a loss position for less than 12 months, gross unrealized losses	(1,950)	(1,570)
Unrealized losses with a loss position for more than 12 months, gross unrealized losses	$ (3,025)	$ (2,957)