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Fair Value Measurements (Financial Assets and Liabilities Measured at Fair Value on Recurring Basis) (Details) - USD ($)
$ in Millions
Nov. 30, 2024
May 31, 2024
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Restricted and unrestricted cash equivalents $ 29.9 $ 417.8
Available-for-sale securities 3,416.9 3,329.6
Other 36.4 33.9
Other long-term liabilities 36.4 33.9
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   386.4
Available-for-sale securities 880.9 852.7
Money Market Securities [Member]    
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Restricted and unrestricted cash equivalents 29.9 31.4
Asset-Backed Securities [Member]    
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Available-for-sale securities 144.5 134.7
Corporate Bonds [Member]    
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Available-for-sale securities 1,401.9 1,368.8
Municipal Bonds [Member]    
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Available-for-sale securities 989.6 973.4
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 29.9 31.4
Other 36.4 33.9
Other long-term liabilities 36.4 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 29.9 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   386.4
Available-for-sale securities 3,416.9 3,329.6
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   386.4
Available-for-sale securities 880.9 852.7
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 144.5 134.7
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]    
Available-for-sale securities 1,401.9 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]    
Available-for-sale securities $ 989.6 $ 973.4