Description of Business, Basis of Presentation, and Significant Accounting Policies (Narrative) (Details) $ in Millions	3 Months Ended		12 Months Ended
	Aug. 31, 2021 USD ($) item segment	Aug. 31, 2020 USD ($) item	May 31, 2022 USD ($)	May 31, 2021 USD ($) item
Accounting Policies [Line Items]
Number of reportable segments | segment	1
Allowance for doubtful accounts	$ 12.1			$ 16.0
Advance collections	4.6			2.5
Maximum individual health insurance claims liability				0.3
Stock-based compensation costs recognized	12.4	$ 13.3
Forecast [Member]
Accounting Policies [Line Items]
Maximum individual health insurance claims liability			$ 0.5
Trade Receivables [Member]
Accounting Policies [Line Items]
Receivables	105.9			98.4
Purchased Receivables [Member]
Accounting Policies [Line Items]
Receivables	$ 480.3			$ 495.9
Accounts And Unbilled Receivables [Member]
Accounting Policies [Line Items]
Number of clients creating a credit concentration | item	0			0
Revenue [Member]
Accounting Policies [Line Items]
Number of clients creating a credit concentration | item	0	0
PEO [Member]
Accounting Policies [Line Items]
Maximum individual workers' compensation claims liability				$ 1.0
PEO [Member] | Forecast [Member]
Accounting Policies [Line Items]
Maximum individual workers' compensation claims liability			1.0
PEO [Member] | Minimum [Member] | Forecast [Member]
Accounting Policies [Line Items]
Individual workers' compensation claims liability			1.0
PEO Aggregate Claims Exceeding $1.0M [Member] | Maximum [Member] | Forecast [Member]
Accounting Policies [Line Items]
Maximum individual workers' compensation claims liability			$ 2.0