Goodwill and Intangible Assets, Net of Accumulated Amortization (Components of Intangible Assets, at Cost) (Details) - USD ($) $ in Millions	Aug. 31, 2018	May 31, 2018
Intangible Assets [Line Items]
Total intangible assets, gross	$ 320.7	$ 321.8
Less: Accumulated amortization	185.2	180.4
Intangible assets, net of accumulated amortization	135.5	141.4	[1]
Client Lists [Member]
Intangible Assets [Line Items]
Total intangible assets, gross	307.4	308.5
Other Intangible Assets [Member]
Intangible Assets [Line Items]
Total intangible assets, gross	$ 13.3	$ 13.3

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