Summary of Securities in Unrealized Loss Position and Included in Accumulated Other Comprehensive (Loss), Aggregated by Length of Time Investments (Detail) - USD ($) $ in Thousands	Feb. 28, 2018	May 31, 2017
Investments Debt And Equity Securities [Abstract]
Total investments with unrealized losses, fair value	$ 93,287	$ 59,987
Unrealized losses with a loss position for less than 12 months, fair value	56,775	40,854
Unrealized losses with a loss position for more than 12 months, fair value	36,512	19,133
Total investments with unrealized losses, gross unrealized losses	(3,837)	(3,505)
Unrealized losses with a loss position for less than 12 months, gross unrealized losses	(795)	(2,983)
Unrealized losses with a loss position for more than 12 months, gross unrealized losses	$ (3,042)	$ (522)