Fair Value Measurements (Financial Instruments on a Recurring Basis) (Details) (Fair Value, Measurements, Recurring, USD $) In Thousands, unless otherwise specified	Feb. 28, 2014	May 31, 2013
Level 1
Fair value on a recurring basis
Cash and cash equivalents	$ 348,859	$ 352,273
Accounts receivable, net		0
Total assets at fair value	348,859	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	4,840	5,719
Current accrued liabilities	365
Total liabilities at fair value	365
Level 2 | Canadian treasury securities
Fair value on a recurring basis
Marketable securities	4,840
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	348,859	352,273
Accounts receivable, net		39
Total assets at fair value	353,699	357,992
Current accrued liabilities	365
Total liabilities at fair value	365
Fair Value | Canadian treasury securities
Fair value on a recurring basis
Marketable securities	4,840
Fair Value | U.S. municipal bonds
Fair value on a recurring basis
Marketable securities		$ 5,680