Fair Value Disclosures - Narrative (Details) - USD ($) $ in Millions	12 Months Ended
	May 31, 2017	May 31, 2016	May 31, 2015
Fair Value Disclosures [Abstract]
Amortized cost basis, marketable securities	$ 22.2	$ 70.4
Marketable securities purchased	$ 171.3	$ 488.8	$ 179.2