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Fair Value Measurements - Fair Value Hierarchy for Financial Assets (Details) - USD ($)
$ in Millions
Mar. 31, 2025
Dec. 31, 2024
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Abstract]    
Cash equivalents $ 362.7 $ 134.2
Debt securities, available-for-sale 1,795.2 1,973.3
Convertible notes receivable 11.1 10.5
Other assets 18.4 20.6
Total assets measured at fair value on a recurring basis 2,187.4 2,138.6
U.S. government agencies    
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Abstract]    
Debt securities, available-for-sale 1,054.7 1,150.1
Commercial paper    
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Abstract]    
Debt securities, available-for-sale 231.3 312.1
Corporate debt    
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Abstract]    
Debt securities, available-for-sale 509.2 511.1
Level 1    
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Abstract]    
Cash equivalents 351.7 134.2
Debt securities, available-for-sale 0.0 0.0
Convertible notes receivable 0.0 0.0
Other assets 18.4 20.6
Total assets measured at fair value on a recurring basis 370.1 154.8
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 11.0 0.0
Debt securities, available-for-sale 1,795.2 1,973.3
Convertible notes receivable 0.0 0.0
Other assets 0.0 0.0
Total assets measured at fair value on a recurring basis 1,806.2 1,973.3
Level 2 | U.S. government agencies    
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Abstract]    
Debt securities, available-for-sale 1,054.7 1,150.1
Level 2 | Commercial paper    
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Abstract]    
Debt securities, available-for-sale 231.3 312.1
Level 2 | Corporate debt    
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Abstract]    
Debt securities, available-for-sale 509.2 511.1
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
Convertible notes receivable 11.1 10.5
Other assets 0.0 0.0
Total assets measured at fair value on a recurring basis 11.1 10.5
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