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Investments - Available-for-Sale Debt Securities (Details) - USD ($)
$ in Millions
Mar. 31, 2024
Dec. 31, 2023
Debt Securities, Available-for-sale [Line Items]    
Amortized Cost $ 2,988.6 $ 3,771.7
Gross Unrealized Gains 1.4 7.4
Gross Unrealized Losses (20.7) (21.9)
Fair Value 2,969.3 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 2,415.1 3,044.5
Gross Unrealized Gains 0.6 5.3
Gross Unrealized Losses (17.1) (17.2)
Fair Value 2,398.6 3,032.6
Corporate debt securities    
Debt Securities, Available-for-sale [Line Items]    
Amortized Cost 573.5 727.2
Gross Unrealized Gains 0.8 2.1
Gross Unrealized Losses (3.6) (4.7)
Fair Value $ 570.7 $ 724.6