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Fair Value Measurements - Investments Measured at Fair Value on Recurring Basis (Details) - USD ($)
$ in Millions
Sep. 30, 2024
Dec. 31, 2023
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Fair Value $ 1,998.6 $ 1,881.0
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 475.8 413.0
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 1,522.8 1,468.0
Cash and cash equivalents    
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Fair Value 349.1 251.1
Cash and cash equivalents | 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 349.1 251.1
Cash and cash equivalents | 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
Available-for-sale debt securities    
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Fair Value 1,522.8 1,468.0
Available-for-sale 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
Available-for-sale 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 1,522.8 1,468.0
Equity investments    
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Fair Value 126.7 161.9
Equity 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 126.7 161.9
Equity 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