Goodwill and Intangible Assets, Net of Accumulated Amortization (Components of Intangible Assets, at Cost) (Details) - USD ($) $ in Millions	Nov. 30, 2018	May 31, 2018
Intangible Assets [Line Items]
Total intangible assets, gross	$ 318.5	$ 321.8
Less: Accumulated amortization	190.5	180.4
Intangible assets, net of accumulated amortization	128.0	141.4	[1]
Client Lists [Member]
Intangible Assets [Line Items]
Total intangible assets, gross	305.7	308.5
Other Intangible Assets [Member]
Intangible Assets [Line Items]
Total intangible assets, gross	$ 12.8	$ 13.3

[1] Amounts have been adjusted to reflect the adoption of ASC Topic 606.