Service Revenue (Summary of PEO Pass-Through Costs Netted in Revenue) (Details) - USD ($) $ in Millions |
12 Months Ended | ||
|---|---|---|---|
May 31, 2026 |
May 31, 2025 |
May 31, 2024 |
|
| Revenue from Contract with Customer [Abstract] | |||
| Payroll wages and payroll taxes | $ 32,197.0 | $ 29,280.0 | $ 27,381.5 |
| State unemployment insurance (included in payroll wages and payroll taxes) | 179.4 | 167.4 | 151.8 |
| Guaranteed cost benefit plans | $ 752.3 | $ 681.6 | $ 660.3 |