Description of Business, Basis of Presentation, and Significant Accounting Policies (Narrative) (Details) $ in Millions	3 Months Ended		6 Months Ended		12 Months Ended
	Nov. 30, 2021 USD ($) item	Nov. 30, 2020 USD ($) item	Nov. 30, 2021 USD ($) item segment	Nov. 30, 2020 USD ($) item	May 31, 2022 USD ($)	May 31, 2021 USD ($) item
Accounting Policies [Line Items]
Number of reportable segments | segment			1
Allowance for credit losses	$ 18.0		$ 18.0			$ 16.0
Advance collections	3.4		3.4			2.5
Maximum individual health insurance claims liability						0.3
Stock-based compensation costs recognized	14.0	$ 12.2	26.4	$ 25.5
Forecast [Member]
Accounting Policies [Line Items]
Maximum individual health insurance claims liability					$ 0.5
Trade Receivables [Member]
Accounting Policies [Line Items]
Receivables	111.8		111.8			98.4
Purchased Receivables [Member]
Accounting Policies [Line Items]
Receivables	$ 723.2		$ 723.2			$ 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	0	0
PEO [Member]
Accounting Policies [Line Items]
Maximum individual workers' compensation claims liability						$ 1.0
PEO [Member] | Forecast [Member]
Accounting Policies [Line Items]
Individual workers' compensation claims liability					1.0
PEO Aggregate Claims Exceeding $1.0M [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