Fair Value Disclosures - Financial Instruments on a Recurring Basis (Details) - USD ($)	May. 31, 2015	May. 31, 2014
Fair value on a recurring basis
Marketable securities		$ 0
Fair Value, Measurements, Recurring
Fair value on a recurring basis
Cash and cash equivalents	$ 417,073,000	513,288,000
Total assets at fair value	433,154,000	513,288,000
Current accrued liabilities		286,000
Total liabilities at fair value		286,000
Fair Value, Measurements, Recurring | Canadian treasury securities
Fair value on a recurring basis
Marketable securities	16,081,000
Fair Value, Measurements, Recurring | Level 1
Fair value on a recurring basis
Cash and cash equivalents	417,073,000	513,288,000
Total assets at fair value	417,073,000	513,288,000
Current accrued liabilities		0
Total liabilities at fair value		0
Fair Value, Measurements, Recurring | Level 1 | Canadian treasury securities
Fair value on a recurring basis
Marketable securities	0
Fair Value, Measurements, Recurring | Level 2
Fair value on a recurring basis
Cash and cash equivalents	0	0
Total assets at fair value	16,081,000	0
Current accrued liabilities		286,000
Total liabilities at fair value		286,000
Fair Value, Measurements, Recurring | Level 2 | Canadian treasury securities
Fair value on a recurring basis
Marketable securities	16,081,000
Fair Value, Measurements, Recurring | Level 3
Fair value on a recurring basis
Cash and cash equivalents	0	0
Total assets at fair value	0	0
Current accrued liabilities		0
Total liabilities at fair value		$ 0
Fair Value, Measurements, Recurring | Level 3 | Canadian treasury securities
Fair value on a recurring basis
Marketable securities	$ 0