Goodwill and Intangible Assets, Net of Accumulated Amortization (Change in Goodwill) (Details) - USD ($) $ in Millions	12 Months Ended
	May 31, 2019		May 31, 2018
Goodwill and Intangible Assets, Net of Accumulated Amortization [Abstract]
Goodwill Balance, beginning of fiscal year	$ 814.0	[1]	$ 657.1
Changes during the period:
Goodwill acquired	972.1		162.6
Acquisition accounting adjustments	1.6
Currency translation adjustment	(5.1)		(5.7)
Goodwill Balance, end of fiscal year	$ 1,782.6		$ 814.0	[1]

[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.