Fair Value Measurements (Financial Assets and Liabilities Measured at Fair Value on Recurring Basis) (Details) - USD ($) $ in Millions	May 31, 2026	May 31, 2025
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Restricted and unrestricted cash equivalents	$ 25.3	$ 663.4
Available-for-sale securities	4,488.4	3,755.5
Other	36.3	34.5
Other long-term liabilities	36.3	34.5
Asset-Backed Securities [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Available-for-sale securities	106.8	159.2
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	2,336.6	1,644.0
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	831.2	973.5
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
Available-for-sale securities	1,213.8	978.8
Money Market Securities [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Restricted and unrestricted cash equivalents	25.3	42.8
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	25.3	42.8
Other	36.3	34.5
Other long-term liabilities	36.3	34.5
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	25.3	42.8
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
Available-for-sale securities	4,488.4	3,755.5
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	106.8	159.2
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]
Restricted and unrestricted cash equivalents		4.7
Available-for-sale securities	2,336.6	1,644.0
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	831.2	973.5
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
Available-for-sale securities	$ 1,213.8	$ 978.8