Fair Value Measurements (Details) (USD $)	Nov. 30, 2011 Y	May 31, 2011
Fair value on a recurring basis
Current accrued liabilities	$ 511,000	$ 869,000
Amortized cost basis of the marketable securities	129,800,000	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	207,772,000	438,106,000
Total assets at fair value	329,601,000	499,248,000
Level 1 | Fair Value, Measurements, Recurring | Canadian treasury securities
Fair value on a recurring basis
Marketable securities:	121,829,000	61,142,000
Level 2 | Fair Value, Measurements, Recurring
Fair value on a recurring basis
Accounts receivable, net	31,000
Total assets at fair value	8,035,000	26,078,000
Current accrued liabilities	511,000	869,000
Total liabilities at fair value	511,000	869,000
Level 2 | Fair Value, Measurements, Recurring | Canadian treasury securities
Fair value on a recurring basis
Marketable securities:	8,004,000	26,078,000
Fair Value | Fair Value, Measurements, Recurring
Fair value on a recurring basis
Cash and cash equivalents	207,772,000	438,106,000
Accounts receivable, net	31,000
Total assets at fair value	337,636,000	525,326,000
Current accrued liabilities	511,000	869,000
Total liabilities at fair value	511,000	869,000
Fair Value | Fair Value, Measurements, Recurring | Canadian treasury securities
Fair value on a recurring basis
Marketable securities:	$ 129,833,000	$ 87,220,000