Accumulated Other Comprehensive Income (Details) - USD ($) $ in Millions	3 Months Ended
	Aug. 31, 2015	Aug. 31, 2014
Equity [Abstract]
Accumulated other comprehensive income, Beginning balance	$ 7.5	$ 21.5
Other comprehensive income:
Unrealized gains on available-for-sale securities, net of tax	6.3	4.6
Reclassification adjustment for the net gain on sale of available-for-sale securities realized in net income, net of tax	0.0	(0.1)
Total other comprehensive income, net of tax	6.3	4.5
Accumulated other comprehensive income, Ending balance	13.8	26.0
Total tax expense included in other comprehensive income	$ 3.6	$ 2.1