Income Taxes - Reconciliation Unrecognized Tax Benefits (Details) - USD ($) $ in Thousands	12 Months Ended
	May 31, 2016	May 31, 2015	May 31, 2014
Reconciliation of Unrecognized Tax Benefits [Roll Forward]
Balance at beginning of period	$ 15,412	$ 13,062	$ 13,709
Additions for tax positions of prior years	3,259	4,001	2,586
Settlements	(48)	(48)	(1,270)
Statute expirations	(2,092)	(1,603)	(1,963)
Balance at end of period	$ 16,531	$ 15,412	$ 13,062