Fair Value Measurements (Financial Assets and Liabilities Measured at Fair Value on Recurring Basis) (Details) - USD ($) $ in Millions	Aug. 31, 2022	May 31, 2022
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Restricted and unrestricted cash equivalents	$ 95.9	$ 219.2
Available-for-sale securities	2,724.8	4,029.2
Other	31.5	30.0
Other long-term liabilities	31.5	29.9
Commercial Paper [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Restricted and unrestricted cash equivalents	49.8	5.2
Time Deposits [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Restricted and unrestricted cash equivalents		187.9
Money Market Securities [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Restricted and unrestricted cash equivalents	46.1	16.1
Asset-Backed Securities [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Available-for-sale securities	54.5	67.2
Corporate Bonds [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Available-for-sale securities	686.9	682.1
Municipal Bonds [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Available-for-sale securities	1,406.8	1,485.8
U.S. Government Agency And Treasury Securities [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Available-for-sale securities	564.9	548.5
Variable Rate Demand Notes [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Restricted and unrestricted cash equivalents		10.0
Available-for-sale securities	11.7	1,245.6
Quoted Prices in Active Markets (Level 1) [Member] | Recurring [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Restricted and unrestricted cash equivalents	46.1	16.1
Other	31.5	30.0
Other long-term liabilities	31.5	29.9
Quoted Prices in Active Markets (Level 1) [Member] | Money Market Securities [Member] | Recurring [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Restricted and unrestricted cash equivalents	46.1	16.1
Significant Other Observable Inputs (Level 2) [Member] | Recurring [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Restricted and unrestricted cash equivalents	49.8	203.1
Available-for-sale securities	2,724.8	4,029.2
Significant Other Observable Inputs (Level 2) [Member] | Commercial Paper [Member] | Recurring [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Restricted and unrestricted cash equivalents	49.8	5.2
Significant Other Observable Inputs (Level 2) [Member] | Time Deposits [Member] | Recurring [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Restricted and unrestricted cash equivalents		187.9
Significant Other Observable Inputs (Level 2) [Member] | Asset-Backed Securities [Member] | Recurring [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Available-for-sale securities	54.5	67.2
Significant Other Observable Inputs (Level 2) [Member] | Corporate Bonds [Member] | Recurring [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Available-for-sale securities	686.9	682.1
Significant Other Observable Inputs (Level 2) [Member] | Municipal Bonds [Member] | Recurring [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Available-for-sale securities	1,406.8	1,485.8
Significant Other Observable Inputs (Level 2) [Member] | U.S. Government Agency And Treasury Securities [Member] | Recurring [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Available-for-sale securities	564.9	548.5
Significant Other Observable Inputs (Level 2) [Member] | Variable Rate Demand Notes [Member] | Recurring [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Restricted and unrestricted cash equivalents		10.0
Available-for-sale securities	$ 11.7	$ 1,245.6