Income Tax - Unrecognized Tax Benefits (Details) (USD $) In Thousands, unless otherwise specified	12 Months Ended
	May 31, 2014	May 31, 2013	May 31, 2012	May 31, 2010
Reconciliation of Unrecognized Tax Benefits, Excluding Amounts Pertaining to Examined Tax Returns [Roll Forward]
Balance at the beginning of the year	$ 53,763	$ 45,595		$ 37,197
Additions based on tax positions related to the current year	8,551	8,778	10,684
Additions for tax positions of prior years	296	142	118
Foreign currency impact for tax positions	5,303	(601)	(2,340)
Reductions for tax positions of prior years	(60)	(151)	(64)
Settlements with taxing authorities	(277)	0	0
Balance at the end of the year	$ 67,576	$ 53,763	$ 45,595	$ 37,197