Fair Value Measurements - Schedule of Fair Value of Financial Instruments Measured on a Recurring Basis (Details) - Fair Value, Measurements, Recurring - USD ($) $ in Thousands	Feb. 28, 2018	May 31, 2017
Fair value on a recurring basis
Cash and cash equivalents	$ 152,563	$ 169,266
Marketable securities	33,693	22,219
Total assets at fair value	186,256	191,485
Level 1
Fair value on a recurring basis
Cash and cash equivalents	152,563	169,266
Marketable securities	0	0
Total assets at fair value	152,563	169,266
Level 2
Fair value on a recurring basis
Cash and cash equivalents	0	0
Marketable securities	33,693	22,219
Total assets at fair value	33,693	22,219
Level 3
Fair value on a recurring basis
Cash and cash equivalents	0	0
Marketable securities	0	0
Total assets at fair value	$ 0	$ 0