Fair Value Disclosures - Financial Instruments on a Recurring Basis (Details) - Fair Value, Measurements, Recurring - USD ($) $ in Thousands	May 31, 2016	May 31, 2015
Fair value on a recurring basis
Cash and cash equivalents	$ 139,357	$ 417,073
Total assets at fair value	209,762	433,154
Total liabilities at fair value	19,628
Canadian treasury securities
Fair value on a recurring basis
Marketable securities	70,405	16,081
Interest rate lock agreement
Fair value on a recurring basis
Interest rate lock agreement	19,628
Level 1
Fair value on a recurring basis
Cash and cash equivalents	139,357	417,073
Total assets at fair value	139,357	417,073
Total liabilities at fair value	0
Level 1 | Canadian treasury securities
Fair value on a recurring basis
Marketable securities	0	0
Level 1 | Interest rate lock agreement
Fair value on a recurring basis
Interest rate lock agreement	0
Level 2
Fair value on a recurring basis
Cash and cash equivalents	0	0
Total assets at fair value	70,405	16,081
Total liabilities at fair value	19,628
Level 2 | Canadian treasury securities
Fair value on a recurring basis
Marketable securities	70,405	16,081
Level 2 | Interest rate lock agreement
Fair value on a recurring basis
Interest rate lock agreement	19,628
Level 3
Fair value on a recurring basis
Cash and cash equivalents	0	0
Total assets at fair value	0	0
Total liabilities at fair value	0
Level 3 | Canadian treasury securities
Fair value on a recurring basis
Marketable securities	0	$ 0
Level 3 | Interest rate lock agreement
Fair value on a recurring basis
Interest rate lock agreement	$ 0