Description of Business, Basis of Presentation, and Significant Accounting Policies (Narrative) (Details) $ in Millions	3 Months Ended		9 Months Ended		12 Months Ended
	Feb. 28, 2023 USD ($) Item	Feb. 28, 2022 USD ($) Item	Feb. 28, 2023 USD ($) Item Segment	Feb. 28, 2022 USD ($) Item	May 31, 2023 USD ($)	May 31, 2022 USD ($) Item
Accounting Policies [Line Items]
Number of reportable segments | Segment			1
Allowance for credit losses	$ 21.1		$ 21.1			$ 18.2
Advance collections	3.9		3.9			2.6
Maximum individual health insurance claims liability						0.5
Stock-based compensation costs recognized	$ 16.3	$ 12.8	$ 46.0	$ 39.2
Accounting Standards Update 2021-10 [Member]
Accounting Policies [Line Items]
Change in accounting principle, accounting standards update, adoption date	Jun. 01, 2022		Jun. 01, 2022
Change in accounting principle, accounting standards update, adopted [true false]	true		true
Change in accounting principle, accounting standards update, immaterial effect [true false]	true		true
Forecast [Member]
Accounting Policies [Line Items]
Maximum individual health insurance claims liability					$ 0.5
Trade Receivables [Member]
Accounting Policies [Line Items]
Receivables	$ 238.5		$ 238.5			123.2
Purchased Receivables [Member]
Accounting Policies [Line Items]
Receivables	$ 582.0		$ 582.0			$ 618.8
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]
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]
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]
Accounting Policies [Line Items]
Maximum individual workers' compensation claims liability						$ 2.0
PEO Aggregate Claims Exceeding $1.0M [Member] | Maximum [Member] | Forecast [Member]
Accounting Policies [Line Items]
Maximum individual workers' compensation claims liability					$ 2.0