Income Taxes - Reconciliation Unrecognized Tax Benefits (Details) - USD ($) $ in Thousands	12 Months Ended
	May. 31, 2015	May. 31, 2014	May. 31, 2013
Reconciliation of Unrecognized Tax Benefits [Roll Forward]
Balance at beginning of period	$ 13,062	$ 13,709	$ 44,531
Additions based on tax positions related to the current year			1,843
Additions for tax positions of prior years	4,001	2,586	2,960
Change in tax regulations			(33,600)
Statute expirations	(1,603)	(1,963)	(2,025)
Settlements	(48)	(1,270)
Balance at end of period	$ 15,412	$ 13,062	$ 13,709