Fair Value Measurements (Details) (USD $)	Aug. 31, 2011 year	May 31, 2011
Fair value on a recurring basis
Amortized cost basis of the marketable securities	$ 126,700,000	$ 87,300,000
Maximum contractual maturities of marketable securities period (in years)	1
Level 1 | Fair Value, Measurements, Recurring
Fair value on a recurring basis
Cash and cash equivalents	150,312,000	438,106,000
Total assets at fair value	249,185,000	499,248,000
Level 1 | Fair Value, Measurements, Recurring | Canadian treasury securities
Fair value on a recurring basis
Marketable securities:	98,873,000	61,142,000
Level 2 | Fair Value, Measurements, Recurring
Fair value on a recurring basis
Total assets at fair value	27,855,000	26,078,000
Current accrued liabilities	548,000	869,000
Total liabilities at fair value	548,000	869,000
Level 2 | Fair Value, Measurements, Recurring | Canadian treasury securities
Fair value on a recurring basis
Marketable securities:	27,855,000	26,078,000
Fair Value | Fair Value, Measurements, Recurring
Fair value on a recurring basis
Cash and cash equivalents	150,312,000	438,106,000
Total assets at fair value	277,040,000	525,326,000
Current accrued liabilities	548,000	869,000
Total liabilities at fair value	548,000	869,000
Fair Value | Fair Value, Measurements, Recurring | Canadian treasury securities
Fair value on a recurring basis
Marketable securities:	$ 126,728,000	$ 87,220,000