Fair Value Measurements (Financial Assets and Liabilities Measured at Fair Value on Recurring Basis) (Details) - USD ($) $ in Millions	May 31, 2025	May 31, 2024
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Restricted and unrestricted cash equivalents	$ 663.4	$ 417.8
Available-for-sale securities	3,755.5	3,329.6
Other	34.5	33.9
Other long-term liabilities	34.5	33.9
Asset-Backed Securities [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Available-for-sale securities	159.2	134.7
Corporate Bonds [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Restricted and unrestricted cash equivalents	4.7
Available-for-sale securities	1,644.0	1,368.8
Municipal Bonds [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Restricted and unrestricted cash equivalents	0.4
Available-for-sale securities	973.5	973.4
U.S. Government Agency And Treasury Securities [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Restricted and unrestricted cash equivalents	615.5	386.4
Available-for-sale securities	978.8	852.7
Money Market Securities [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Restricted and unrestricted cash equivalents	42.8	31.4
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	42.8	31.4
Other	34.5	33.9
Other long-term liabilities	34.5	33.9
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	42.8	31.4
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	620.6	386.4
Available-for-sale securities	3,755.5	3,329.6
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	159.2	134.7
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]
Restricted and unrestricted cash equivalents	4.7
Available-for-sale securities	1,644.0	1,368.8
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]
Restricted and unrestricted cash equivalents	0.4
Available-for-sale securities	973.5	973.4
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]
Restricted and unrestricted cash equivalents	615.5	386.4
Available-for-sale securities	$ 978.8	$ 852.7