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Fair Value Measurements - Schedule of Fair Value Hierarchy for Financial Assets (Details) - USD ($)
$ in Millions
Dec. 31, 2023
Dec. 31, 2022
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Cash equivalents $ 315.9 $ 420.7
Debt securities, available-for-sale 2,157.8 1,813.9
Other assets 15.2 10.2
Total assets measured at fair value on a recurring basis 2,488.9 2,244.8
Level 1    
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Cash equivalents 315.9 375.9
Debt securities, available-for-sale 0.0 0.0
Other assets 15.2 10.2
Total assets measured at fair value on a recurring basis 331.1 386.1
Level 2    
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Cash equivalents 0.0 44.8
Debt securities, available-for-sale 2,157.8 1,813.9
Other assets 0.0 0.0
Total assets measured at fair value on a recurring basis 2,157.8 1,858.7
Level 3    
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
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
U.S. government agencies    
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Debt securities, available-for-sale 1,612.5 1,530.7
U.S. government agencies | Level 1    
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Debt securities, available-for-sale 0.0 0.0
U.S. government agencies | Level 2    
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Debt securities, available-for-sale 1,612.5 1,530.7
U.S. government agencies | Level 3    
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Debt securities, available-for-sale 0.0 0.0
Commercial paper    
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Debt securities, available-for-sale 184.7 119.4
Commercial paper | Level 1    
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Debt securities, available-for-sale 0.0 0.0
Commercial paper | Level 2    
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Debt securities, available-for-sale 184.7 119.4
Commercial paper | Level 3    
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Debt securities, available-for-sale 0.0 0.0
Corporate debt    
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Debt securities, available-for-sale 360.6 163.8
Corporate debt | Level 1    
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Debt securities, available-for-sale 0.0 0.0
Corporate debt | Level 2    
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Debt securities, available-for-sale 360.6 163.8
Corporate debt | Level 3    
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Debt securities, available-for-sale $ 0.0 $ 0.0