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, 2020 USD ($)	Nov. 30, 2019 USD ($)	Nov. 30, 2020 USD ($) segment	Nov. 30, 2019 USD ($)	May 31, 2021 USD ($)	May 31, 2020 USD ($)
Accounting Policies [Line Items]
Number of reportable segments | segment			1
Allowance for doubtful accounts	$ 13.8		$ 13.8			$ 12.5
Advance collections	21.0		21.0			6.1
Maximum individual health insurance claims liability						0.3
Stock-based compensation expense	12.2	$ 13.3	25.5	$ 23.8
Forecast [Member]
Accounting Policies [Line Items]
Maximum individual health insurance claims liability					$ 0.3
Trade Receivables [Member]
Accounting Policies [Line Items]
Receivables	100.0		100.0			84.7
Purchased Receivables [Member]
Accounting Policies [Line Items]
Receivables	$ 432.8		$ 432.8			311.9
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