Share-based Awards and Options - Employee Stock Purchase Plan (Details) (USD $)	12 Months Ended
	May 31, 2014	May 31, 2013	May 31, 2012
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Share-based compensation expense	$ 29,794,000	$ 18,427,000	$ 16,391,000
Employee Stock Purchase Plan [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Common stock authorized (in shares)	2,400,000
Maximum employee subscription amount	25,000
Maximum employee subscription rate	20.00%
Discounted market value	85.00%
Shares issued in period (in shares)	1,100,000
Shares reserved for future issuance (in shares)	1,300,000
Share-based compensation expense	$ 600,000	$ 600,000	$ 500,000
Weighted average grant-date fair value (in dollars per share)	$ 7	$ 6	$ 7
Discount from market price	15.00%