Fair Value Measurements (Financial Instruments on a Recurring Basis) (Details) (Fair Value, Measurements, Recurring [Member], USD $) In Thousands, unless otherwise specified	Aug. 31, 2013	May 31, 2013
Level 1 [Member]
Fair value on a recurring basis
Cash and cash equivalents	$ 278,670	$ 352,273
Accounts receivable, net		0
Total assets at fair value	278,670	352,273
Current accrued liabilities	0
Total liabilities at fair value	0
Level 1 [Member] | Canadian treasury securities [Member]
Fair value on a recurring basis
Marketable securities:	0
Level 1 [Member] | U.S. municipal bonds [Member]
Fair value on a recurring basis
Marketable securities:		0
Level 2 [Member]
Fair value on a recurring basis
Cash and cash equivalents	0	0
Accounts receivable, net		39
Total assets at fair value	4,189	5,719
Current accrued liabilities	107
Total liabilities at fair value	107
Level 2 [Member] | Canadian treasury securities [Member]
Fair value on a recurring basis
Marketable securities:	4,189
Level 2 [Member] | U.S. municipal bonds [Member]
Fair value on a recurring basis
Marketable securities:		5,680
Level 3 [Member]
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 [Member] | Canadian treasury securities [Member]
Fair value on a recurring basis
Marketable securities:	0
Level 3 [Member] | U.S. municipal bonds [Member]
Fair value on a recurring basis
Marketable securities:		0
Fair Value [Member]
Fair value on a recurring basis
Cash and cash equivalents	278,670	352,273
Accounts receivable, net		39
Total assets at fair value	282,859	357,992
Current accrued liabilities	107
Total liabilities at fair value	107
Fair Value [Member] | Canadian treasury securities [Member]
Fair value on a recurring basis
Marketable securities:	4,189
Fair Value [Member] | U.S. municipal bonds [Member]
Fair value on a recurring basis
Marketable securities:		$ 5,680