Service Revenue (Summary of PEO Pass-Through Costs Netted in Revenue) (Details) - USD ($) $ in Millions	12 Months Ended
	May 31, 2021	May 31, 2020	May 31, 2019
Service Revenue [Abstract]
Payroll wages and payroll taxes	$ 20,600.0	$ 20,500.0	$ 14,500.0
State unemployment insurance (included in payroll wages and payroll taxes)	119.0	85.7	82.3
Guaranteed cost benefit plans	$ 586.4	$ 647.0	$ 451.8