Fair Value Measurements - Schedule of Fair Value of Financial Instruments Measured on a Recurring Basis (Details) - Fair Value, Measurements, Recurring - USD ($) $ in Thousands	Aug. 31, 2017	May 31, 2017
Fair value on a recurring basis
Cash and cash equivalents	$ 191,414	$ 169,266
Total assets at fair value	213,040	191,485
Canadian treasury securities
Fair value on a recurring basis
Marketable securities	21,626	22,219
Level 1
Fair value on a recurring basis
Cash and cash equivalents	191,414	169,266
Total assets at fair value	191,414	169,266
Level 1 | Canadian treasury securities
Fair value on a recurring basis
Marketable securities	0	0
Level 2
Fair value on a recurring basis
Cash and cash equivalents	0	0
Total assets at fair value	21,626	22,219
Level 2 | Canadian treasury securities
Fair value on a recurring basis
Marketable securities	21,626	22,219
Level 3
Fair value on a recurring basis
Cash and cash equivalents	0	0
Total assets at fair value	0	0
Level 3 | Canadian treasury securities
Fair value on a recurring basis
Marketable securities	$ 0	$ 0