Fair Value Measurements - Schedule of Fair Value of Financial Instruments Measured on a Recurring Basis (Details) - Fair Value, Measurements, Recurring - USD ($) $ in Thousands	Nov. 30, 2017	May 31, 2017
Fair value on a recurring basis
Cash and cash equivalents	$ 236,002	$ 169,266
Total assets at fair value	258,734	191,485
Canadian treasury securities
Fair value on a recurring basis
Marketable securities	22,732	22,219
Level 1
Fair value on a recurring basis
Cash and cash equivalents	236,002	169,266
Total assets at fair value	236,002	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	22,732	22,219
Level 2 | Canadian treasury securities
Fair value on a recurring basis
Marketable securities	22,732	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