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Fair Value Measurements (Financial Assets and Liabilities Measured at Fair Value on Recurring Basis) (Details) - USD ($) $ in Millions | Aug. 31, 2024 | May 31, 2024
--- | --- | ---
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |
Restricted and unrestricted cash equivalents | $ 391.8 | $ 417.8
Available-for-sale securities | 3,431.3 | 3,329.6
Other | 36.7 | 33.9
Other long-term liabilities | 36.7 | 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 | 374.9 |
Available-for-sale securities | 878.2 | 852.7
U.S. Government Agency Securities [Member] |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |
Restricted and unrestricted cash equivalents |  | 386.4
Money Market Securities [Member] |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |
Restricted and unrestricted cash equivalents | 16.9 | 31.4
Asset-Backed Securities [Member] |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |
Available-for-sale securities | 133.1 | 134.7
Corporate Bonds [Member] |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |
Available-for-sale securities | 1,421.1 | 1,368.8
Municipal Bonds [Member] |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |
Available-for-sale securities | 998.9 | 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 | 16.9 | 31.4
Other | 36.7 | 33.9
Other long-term liabilities | 36.7 | 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 | 16.9 | 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 | 374.9 | 386.4
Available-for-sale securities | 3,431.3 | 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 | 374.9 |
Available-for-sale securities | 878.2 | 852.7
Significant Other Observable Inputs (Level 2) [Member] | U.S. Government Agency Securities [Member] | Recurring [Member] |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |
Restricted and unrestricted cash equivalents |  | 386.4
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 | 133.1 | 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 | 1,421.1 | 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 | $ 998.9 | $ 973.4

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