Accumulated Other Comprehensive Income (Details) - USD ($) $ in Millions	12 Months Ended
	May 31, 2016	May 31, 2015	May 31, 2014
Accumulated Other Comprehensive Income [Abstract]
Beginning balance	$ 7.5	$ 21.5	$ 22.0
Unrealized holding gains/(losses)	34.2	(21.5)	0.3
Income tax (expense)/benefit related to unrealized holding gains/(losses)	(12.4)	7.7	(0.4)
Reclassification adjustment for the net gain on sale of available-for-sale securities realized in net income	(0.1)	(0.3)	(0.6)
Income tax expense on reclassification adjustment for the net gain on sale of available-for-sale securities		0.1	0.2
Total other comprehensive income/(loss), net of tax	21.7	(14.0)	(0.5)
Ending balance	29.2	7.5	21.5
Total tax expense/(benefit) included in other comprehensive income/(loss)	$ 12.4	$ (7.8)	$ 0.2