Valuation and Qualifying Accounts (Details) - USD ($) $ in Millions	12 Months Ended
	May. 31, 2015	May. 31, 2014	May. 31, 2013
Allowance for Doubtful Accounts [Member]
Movement in Valuation Allowances and Reserves [Roll Forward]
Balance as of beginning of fiscal year	$ 1.5	$ 1.0	$ 1.2
Additions charged to expenses	1.6	2.5	1.7
Additions to/(deductions from) other accounts	0.1	0.0	0.0
Costs and deductions	1.8	2.0	1.9
Balance as of end of fiscal year	1.4	1.5	1.0
Reserve For Client Fund Losses [Member]
Movement in Valuation Allowances and Reserves [Roll Forward]
Balance as of beginning of fiscal year	1.9	2.4	2.1
Additions charged to expenses	2.0	2.2	2.7
Additions to/(deductions from) other accounts	(0.2)	0.0	0.0
Costs and deductions	1.9	2.7	2.4
Balance as of end of fiscal year	$ 1.8	$ 1.9	$ 2.4