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Investments - Available-for-Sale Debt Securities (Details) - USD ($)
$ in Millions
Sep. 30, 2024
Dec. 31, 2023
Debt Securities, Available-for-sale [Line Items]    
Amortized Cost $ 3,016.7 $ 3,771.7
Gross Unrealized Gains 16.2 7.4
Gross Unrealized Losses (2.2) (21.9)
Fair Value 3,030.7 3,757.2
Available-for-sale debt securities receivable   21.0
U.S. government and agency securities    
Debt Securities, Available-for-sale [Line Items]    
Amortized Cost 2,413.9 3,044.5
Gross Unrealized Gains 11.4 5.3
Gross Unrealized Losses (1.5) (17.2)
Fair Value 2,423.8 3,032.6
Corporate debt securities    
Debt Securities, Available-for-sale [Line Items]    
Amortized Cost 602.8 727.2
Gross Unrealized Gains 4.8 2.1
Gross Unrealized Losses (0.7) (4.7)
Fair Value $ 606.9 $ 724.6