Accumulated Other Comprehensive Income (Changes in Accumulated Other Comprehensive Income) (Details) - USD ($) $ in Millions	12 Months Ended
	May 31, 2017	May 31, 2016	May 31, 2015
Accumulated Other Comprehensive Income [Abstract]
Beginning balance	$ 29.2	$ 7.5	$ 21.5
Unrealized holding (losses)/gains	(14.7)	34.2	(21.5)
Income tax benefit/(expense) related to unrealized holding (losses)/gains	5.6	(12.4)	7.7
Reclassification adjustment for the net gain on sale of available-for-sale securities realized in net income	(0.1)	(0.1)	(0.3)
Income tax expense on reclassification adjustment for the net gain on sale of available-for-sale securities			0.1
Total other comprehensive (loss)/income, net of tax	(9.2)	21.7	(14.0)
Ending balance	20.0	29.2	7.5
Total tax (benefit)/expense included in other comprehensive (loss)/income	$ (5.6)	$ 12.4	$ (7.8)