Fair Value Measurements (Financial Instruments on a Recurring Basis) (Details) (Fair Value, Measurements, Recurring, USD $) In Thousands, unless otherwise specified	Nov. 30, 2013	May 31, 2013
Level 1
Fair value on a recurring basis
Cash and cash equivalents	$ 309,334	$ 352,273
Accounts receivable, net		0
Total assets at fair value	309,334	352,273
Current accrued liabilities	0
Total liabilities at fair value	0
Level 1 | Canadian treasury securities
Fair value on a recurring basis
Marketable securities	0
Level 1 | U.S. municipal bonds
Fair value on a recurring basis
Marketable securities		0
Level 2
Fair value on a recurring basis
Cash and cash equivalents	0	0
Accounts receivable, net		39
Total assets at fair value	4,156	5,719
Current accrued liabilities	237
Total liabilities at fair value	237
Level 2 | Canadian treasury securities
Fair value on a recurring basis
Marketable securities	4,156
Level 2 | U.S. municipal bonds
Fair value on a recurring basis
Marketable securities		5,680
Level 3
Fair value on a recurring basis
Cash and cash equivalents	0	0
Accounts receivable, net		0
Total assets at fair value	0	0
Current accrued liabilities	0
Total liabilities at fair value	0
Level 3 | Canadian treasury securities
Fair value on a recurring basis
Marketable securities	0
Level 3 | U.S. municipal bonds
Fair value on a recurring basis
Marketable securities		0
Fair Value
Fair value on a recurring basis
Cash and cash equivalents	309,334	352,273
Accounts receivable, net		39
Total assets at fair value	313,490	357,992
Current accrued liabilities	237
Total liabilities at fair value	237
Fair Value | Canadian treasury securities
Fair value on a recurring basis
Marketable securities	4,156
Fair Value | U.S. municipal bonds
Fair value on a recurring basis
Marketable securities		$ 5,680