Goodwill and Intangible Assets, Net of Accumulated Amortization (Components of Intangible Assets, at Cost) (Details) - USD ($) $ in Millions	Nov. 30, 2017	May 31, 2017
Finite-Lived Intangible Assets [Line Items]
Total intangible assets, gross	$ 283.3	$ 298.9
Less: Accumulated amortization	205.9	241.3
Intangible assets, net of accumulated amortization	77.4	57.6
Client Lists [Member]
Finite-Lived Intangible Assets [Line Items]
Total intangible assets, gross	277.9	293.5
Other Intangible Assets [Member]
Finite-Lived Intangible Assets [Line Items]
Total intangible assets, gross	$ 5.4	$ 5.4