Summary of Securities in Unrealized Loss Position and Included in Accumulated Other Comprehensive (Loss), Aggregated by Length of Time Investments (Detail) - USD ($) $ in Thousands	Nov. 30, 2017	May 31, 2017
Investments Debt And Equity Securities [Abstract]
Total investments with unrealized losses, fair value	$ 55,199	$ 59,987
Unrealized losses with a loss position for less than 12 months, fair value	13,813	40,854
Unrealized losses with a loss position for more than 12 months, fair value	41,386	19,133
Total investments with unrealized losses, gross unrealized losses	(2,602)	(3,505)
Unrealized losses with a loss position for less than 12 months, gross unrealized losses	(75)	(2,983)
Unrealized losses with a loss position for more than 12 months, gross unrealized losses	$ (2,527)	$ (522)