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Debt Securities - Gross Unrealized Losses and Fair Value Available-For-Sale Investments in Unrealized Loss Position (Details) - USD ($)
$ in Millions
Mar. 31, 2024
Dec. 31, 2023
Corporate debt securities    
Debt Securities, Available-for-sale [Line Items]    
Less Than 12 Months, Fair Value $ 555.2 $ 265.1
Less Than 12 Months, Unrealized Loss (1.6) (0.4)
12 Months or Longer, Fair Value 90.0 183.8
12 Months or Longer, Unrealized Loss (0.6) (1.0)
Total, Fair Value 645.2 448.9
Total, Unrealized Loss (2.2) (1.4)
Securities of government-sponsored entities    
Debt Securities, Available-for-sale [Line Items]    
Less Than 12 Months, Fair Value 383.3 214.6
Less Than 12 Months, Unrealized Loss (0.9) (0.2)
12 Months or Longer, Fair Value 16.8 16.7
12 Months or Longer, Unrealized Loss (0.3) (0.4)
Total, Fair Value 400.1 231.3
Total, Unrealized Loss $ (1.2) $ (0.6)