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Fair Value Measurements - Fair Value Hierarchy for Financial Assets (Details) - USD ($) $ in Millions | Jun. 30, 2024 | Dec. 31, 2023
--- | --- | ---
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Abstract] |  |
Cash equivalents | $ 581.7 | $ 315.9
Debt securities, available-for-sale | 2,181.5 | 2,157.8
Other assets | 19.9 | 15.2
Total assets measured at fair value on a recurring basis | 2,783.1 | 2,488.9
U.S. government agencies |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Abstract] |  |
Debt securities, available-for-sale | 1,233.3 | 1,612.5
Commercial paper |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Abstract] |  |
Debt securities, available-for-sale | 497.3 | 184.7
Corporate debt |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Abstract] |  |
Debt securities, available-for-sale | 450.9 | 360.6
Level 1 |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Abstract] |  |
Cash equivalents | 543.2 | 315.9
Debt securities, available-for-sale | 0.0 | 0.0
Other assets | 19.9 | 15.2
Total assets measured at fair value on a recurring basis | 563.1 | 331.1
Level 1 | U.S. government agencies |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Abstract] |  |
Debt securities, available-for-sale | 0.0 | 0.0
Level 1 | Commercial paper |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Abstract] |  |
Debt securities, available-for-sale | 0.0 | 0.0
Level 1 | Corporate debt |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Abstract] |  |
Debt securities, available-for-sale | 0.0 | 0.0
Level 2 |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Abstract] |  |
Cash equivalents | 38.5 | 0.0
Debt securities, available-for-sale | 2,181.5 | 2,157.8
Other assets | 0.0 | 0.0
Total assets measured at fair value on a recurring basis | 2,220.0 | 2,157.8
Level 2 | U.S. government agencies |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Abstract] |  |
Debt securities, available-for-sale | 1,233.3 | 1,612.5
Level 2 | Commercial paper |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Abstract] |  |
Debt securities, available-for-sale | 497.3 | 184.7
Level 2 | Corporate debt |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Abstract] |  |
Debt securities, available-for-sale | 450.9 | 360.6
Level 3 |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Abstract] |  |
Cash equivalents | 0.0 | 0.0
Debt securities, available-for-sale | 0.0 | 0.0
Other assets | 0.0 | 0.0
Total assets measured at fair value on a recurring basis | 0.0 | 0.0
Level 3 | U.S. government agencies |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Abstract] |  |
Debt securities, available-for-sale | 0.0 | 0.0
Level 3 | Commercial paper |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Abstract] |  |
Debt securities, available-for-sale | 0.0 | 0.0
Level 3 | Corporate debt |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Abstract] |  |
Debt securities, available-for-sale | $ 0.0 | $ 0.0

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