Fair Value Disclosures (Financial Instruments on a Recurring Basis) (Details) (Fair Value, Measurements, Recurring [Member], USD $) In Thousands, unless otherwise specified	May 31, 2014	May 31, 2013
Level 1 [Member]
Fair value on a recurring basis
Cash and cash equivalents	$ 513,288	$ 352,273
Accounts Receivable, Fair Value Disclosure		0
Total assets at fair value	513,288	352,273
Current accrued liabilities	0
Total liabilities at fair value	0
Level 2 [Member]
Fair value on a recurring basis
Cash and cash equivalents	0	0
Accounts Receivable, Fair Value Disclosure		39
Total assets at fair value	0	5,719
Current accrued liabilities	286
Total liabilities at fair value	286
Level 3 [Member]
Fair value on a recurring basis
Cash and cash equivalents	0	0
Accounts Receivable, Fair Value Disclosure		0
Total assets at fair value	0	0
Current accrued liabilities	0
Total liabilities at fair value	0
Fair Value [Member]
Fair value on a recurring basis
Cash and cash equivalents	513,288	352,273
Accounts Receivable, Fair Value Disclosure		39
Total assets at fair value	513,288	357,992
Current accrued liabilities	286
Total liabilities at fair value	286
U.S. municipal bonds [Member] | Level 1 [Member]
Fair value on a recurring basis
Available-for-sale Securities		0
U.S. municipal bonds [Member] | Level 2 [Member]
Fair value on a recurring basis
Available-for-sale Securities		5,680
U.S. municipal bonds [Member] | Level 3 [Member]
Fair value on a recurring basis
Available-for-sale Securities		0
U.S. municipal bonds [Member] | Fair Value [Member]
Fair value on a recurring basis
Available-for-sale Securities		$ 5,680