Goodwill and Intangible Assets, Net of Accumulated Amortization (Components of Intangible Assets, at Cost) (Details) - USD ($) $ in Millions	May 31, 2017	May 31, 2016
Finite-Lived Intangible Assets [Line Items]
Total intangible assets, gross	$ 298.9	$ 294.6
Less: Accumulated amortization	241.3	225.1
Intangible assets, net of accumulated amortization	57.6	69.5
Client Lists [Member]
Finite-Lived Intangible Assets [Line Items]
Total intangible assets, gross	293.5	289.2
Other Intangible Assets [Member]
Finite-Lived Intangible Assets [Line Items]
Total intangible assets, gross	$ 5.4	$ 5.4