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Fair Value Measurements - Fair Value Hierarchy for Financial Assets (Details) - USD ($) $ in Millions | Sep. 30, 2023 | Dec. 31, 2022
--- | --- | ---
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Abstract] |  |
Cash equivalents | $ 390.8 | $ 420.7
Debt securities, available-for-sale | 2,596.6 | 1,813.9
Other assets | 13.7 | 10.2
Total assets measured at fair value on a recurring basis | 3,001.1 | 2,244.8
U.S. government agencies |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Abstract] |  |
Debt securities, available-for-sale | 1,819.0 | 1,530.7
Commercial paper |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Abstract] |  |
Debt securities, available-for-sale | 387.3 | 119.4
Corporate debt |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Abstract] |  |
Debt securities, available-for-sale | 390.3 | 163.8
Level 1 |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Abstract] |  |
Cash equivalents | 360.9 | 375.9
Debt securities, available-for-sale | 0.0 | 0.0
Other assets | 13.7 | 10.2
Total assets measured at fair value on a recurring basis | 374.6 | 386.1
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 | 29.9 | 44.8
Debt securities, available-for-sale | 2,596.6 | 1,813.9
Other assets | 0.0 | 0.0
Total assets measured at fair value on a recurring basis | 2,626.5 | 1,858.7
Level 2 | U.S. government agencies |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Abstract] |  |
Debt securities, available-for-sale | 1,819.0 | 1,530.7
Level 2 | Commercial paper |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Abstract] |  |
Debt securities, available-for-sale | 387.3 | 119.4
Level 2 | Corporate debt |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Abstract] |  |
Debt securities, available-for-sale | 390.3 | 163.8
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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