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Fair Value Measurements - Investments Measured at Fair Value on Recurring Basis (Details) - USD ($) $ in Millions | Jun. 30, 2024 | Dec. 31, 2023
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Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |
Fair Value | $ 1,828.3 | $ 1,889.0
Fair Value, Recurring | Level 1 |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |
Fair value of assets on recurring basis | 291.3 | 421.0
Fair Value, Recurring | Level 2 |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |
Fair value of assets on recurring basis | 1,537.0 | 1,468.0
Corporate debt securities |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |
Fair Value | 1,004.9 | 867.2
Corporate debt securities | Fair Value, Recurring | Level 1 |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |
Fair value of assets on recurring basis | 0.0 | 0.0
Corporate debt securities | Fair Value, Recurring | Level 2 |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |
Fair value of assets on recurring basis | 1,004.9 | 867.2
Securities of government-sponsored entities |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |
Fair Value | 474.9 | 547.3
Securities of government-sponsored entities | Fair Value, Recurring | Level 1 |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |
Fair value of assets on recurring basis | 0.0 | 0.0
Securities of government-sponsored entities | Fair Value, Recurring | Level 2 |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |
Fair value of assets on recurring basis | 474.9 | 547.3
Equity security investments |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |
Fair Value | 143.6 | 161.9
Equity security investments | Fair Value, Recurring | Level 1 |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |
Fair value of assets on recurring basis | 143.6 | 161.9
Equity security investments | Fair Value, Recurring | Level 2 |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |
Fair value of assets on recurring basis | 0.0 | 0.0
Cash and money market funds |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |
Fair Value | 139.7 | 251.1
Cash and money market funds | Fair Value, Recurring | Level 1 |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |
Fair value of assets on recurring basis | 139.7 | 251.1
Cash and money market funds | Fair Value, Recurring | Level 2 |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |
Fair value of assets on recurring basis | 0.0 | 0.0
Restricted cash |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |
Fair Value | 8.0 | 8.0
Restricted cash | Fair Value, Recurring | Level 1 |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |
Fair value of assets on recurring basis | 8.0 | 8.0
Restricted cash | Fair Value, Recurring | Level 2 |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |
Fair value of assets on recurring basis | 0.0 | 0.0
Commercial paper |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |
Fair Value | 57.2 | 53.5
Commercial paper | Fair Value, Recurring | Level 1 |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |
Fair value of assets on recurring basis | 0.0 | 0.0
Commercial paper | Fair Value, Recurring | Level 2 |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |
Fair value of assets on recurring basis | $ 57.2 | $ 53.5

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