Goodwill and Intangible Assets, Net of Accumulated Amortization (Components of Intangible Assets, at Cost) (Details) - USD ($) $ in Millions		May 31, 2019	May 31, 2018
Intangible Assets [Line Items]
Total intangible assets, gross		$ 631.2	$ 321.8
Less: Accumulated amortization		232.1	180.4
Intangible assets, net of accumulated amortization		399.1	141.4	[1]
Other Intangible Assets [Member]
Intangible Assets [Line Items]
Total intangible assets, gross		23.0	13.3
Client Lists [Member]
Intangible Assets [Line Items]
Total intangible assets, gross	[2]	$ 608.2	$ 308.5

[1] Amounts have been adjusted to reflect the adoption of ASC Topic 606 and to provide further detail related to certain professional employer organization ("PEO") balances.

[2] Client lists and other intangible assets include current estimates of amounts acquired from Oasis as of December 20, 2018. Refer to Note D of this Item 8 for further details.