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	Aug. 31, 2015	May. 31, 2015
Schedule of Available-for-sale Securities [Line Items]
Total investments with unrealized losses, fair value	$ 100,639	$ 58,978
Unrealized losses with a loss position for less than 12 months, fair value	81,271	32,693
Unrealized losses with a loss position for more than 12 months, fair value	19,368	26,285
Total investments with unrealized losses, gross unrealized losses	(9,777)	(3,815)
Unrealized losses with a loss position for less than 12 months, gross unrealized losses	(7,231)	(1,441)
Unrealized losses with a loss position for more than 12 months, gross unrealized losses	$ (2,546)	$ (2,374)