Summary of Securities in Unrealized Loss Position and Included in Accumulated Other Comprehensive Income (Loss), Aggregated by Length of Time Investments (Detail) - USD ($) $ in Thousands	Feb. 29, 2016	May. 31, 2015
Investments Debt And Equity Securities [Abstract]
Total investments with unrealized losses, fair value	$ 98,332	$ 58,978
Unrealized losses with a loss position for less than 12 months, fair value	50,214	32,693
Unrealized losses with a loss position for more than 12 months, fair value	48,118	26,285
Total investments with unrealized losses, gross unrealized losses	(17,040)	(3,815)
Unrealized losses with a loss position for less than 12 months, gross unrealized losses	(9,564)	(1,441)
Unrealized losses with a loss position for more than 12 months, gross unrealized losses	$ (7,476)	$ (2,374)