Description of Business, Basis of Presentation, and Significant Accounting Policies (Narrative) (Details) $ in Millions	3 Months Ended		12 Months Ended
	Aug. 31, 2020 USD ($) segment	Aug. 31, 2019 USD ($)	May 31, 2021 USD ($)	May 31, 2020 USD ($)
Accounting Policies [Line Items]
Number of reportable segments | segment	1
Allowance for doubtful accounts	$ 11.5			$ 12.5
Advance collections	20.1			6.1
Maximum individual health insurance claims liability				0.3
Stock-based compensation expense	13.3	$ 10.5
Forecast [Member]
Accounting Policies [Line Items]
Maximum individual health insurance claims liability			$ 0.3
Trade Receivables [Member]
Accounting Policies [Line Items]
Receivables	89.8			84.7
Purchased Receivables [Member]
Accounting Policies [Line Items]
Receivables	$ 336.5			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