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Fair Value Measurements - Assets Measured at Fair Value on Recurring Basis (Details) - Fair Value Measurements Recurring - USD ($) $ in Millions | Dec. 31, 2023 | Dec. 31, 2022
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Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |
Fair value | $ 1,889.0 | $ 1,398.6
Corporate debt securities |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |
Fair value | 867.2 | 548.4
Securities of government-sponsored entities |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |
Fair value | 547.3 | 321.4
Equity securities |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |
Fair value | 161.9 | 102.1
Cash and money market funds |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |
Fair value | 251.1 | 262.9
Restricted cash |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |
Fair value | 8.0 | 7.8
Commercial paper |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |
Fair value | 53.5 | 156.0
Level 1 |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |
Fair value of assets on recurring basis | 421.0 | 372.8
Level 1 | Corporate debt securities |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |
Fair value of assets on recurring basis | 0.0 | 0.0
Level 1 | Securities of government-sponsored entities |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |
Fair value of assets on recurring basis | 0.0 | 0.0
Level 1 | Equity securities |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |
Fair value of assets on recurring basis | 161.9 | 102.1
Level 1 | Cash and money market funds |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |
Fair value of assets on recurring basis | 251.1 | 262.9
Level 1 | Restricted cash |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |
Fair value of assets on recurring basis | 8.0 | 7.8
Level 1 | Commercial paper |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |
Fair value of assets on recurring basis | 0.0 | 0.0
Level 2 |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |
Fair value of assets on recurring basis | 1,468.0 | 1,025.8
Level 2 | Corporate debt securities |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |
Fair value of assets on recurring basis | 867.2 | 548.4
Level 2 | Securities of government-sponsored entities |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |
Fair value of assets on recurring basis | 547.3 | 321.4
Level 2 | Equity securities |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |
Fair value of assets on recurring basis | 0.0 | 0.0
Level 2 | Cash and money market funds |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |
Fair value of assets on recurring basis | 0.0 | 0.0
Level 2 | Restricted cash |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |
Fair value of assets on recurring basis | 0.0 | 0.0
Level 2 | Commercial paper |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |
Fair value of assets on recurring basis | $ 53.5 | $ 156.0

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