Fair Value Measurements - Financial Instruments on a Recurring Basis (Details) - USD ($) $ in Thousands	Aug. 31, 2015	May. 31, 2015
Fair value on a recurring basis
Marketable securities	$ 53,354	$ 16,081
Fair Value, Measurements, Recurring
Fair value on a recurring basis
Cash and cash equivalents	146,860	417,073
Total assets at fair value	200,214	433,154
Level 1 | Fair Value, Measurements, Recurring
Fair value on a recurring basis
Cash and cash equivalents	146,860	417,073
Total assets at fair value	146,860	417,073
Level 2 | Fair Value, Measurements, Recurring
Fair value on a recurring basis
Cash and cash equivalents	0	0
Total assets at fair value	53,354	16,081
Level 3 | Fair Value, Measurements, Recurring
Fair value on a recurring basis
Cash and cash equivalents	0	0
Total assets at fair value	0	0
Canadian treasury securities | Fair Value, Measurements, Recurring
Fair value on a recurring basis
Marketable securities	53,354	16,081
Canadian treasury securities | Level 1 | Fair Value, Measurements, Recurring
Fair value on a recurring basis
Marketable securities	0	0
Canadian treasury securities | Level 2 | Fair Value, Measurements, Recurring
Fair value on a recurring basis
Marketable securities	53,354	16,081
Canadian treasury securities | Level 3 | Fair Value, Measurements, Recurring
Fair value on a recurring basis
Marketable securities	$ 0	$ 0