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Debt Securities - Gross Unrealized Losses and Fair Value Available-For-Sale Investments in Unrealized Loss Position (Details) - USD ($)
$ in Millions
Sep. 30, 2023
Dec. 31, 2022
Corporate debt securities    
Debt Securities, Available-for-sale [Line Items]    
Less Than 12 Months, Fair Value $ 386.0 $ 199.5
Less Than 12 Months, Unrealized Loss (2.8) (1.9)
12 Months or Longer, Fair Value 224.4 299.1
12 Months or Longer, Unrealized Loss (2.2) (5.6)
Total, Fair Value 610.4 498.6
Total, Unrealized Loss (5.0) (7.5)
Securities of government-sponsored entities    
Debt Securities, Available-for-sale [Line Items]    
Less Than 12 Months, Fair Value 375.7 107.7
Less Than 12 Months, Unrealized Loss (1.2) (2.5)
12 Months or Longer, Fair Value 93.5 198.4
12 Months or Longer, Unrealized Loss (1.2) (4.5)
Total, Fair Value 469.2 306.1
Total, Unrealized Loss $ (2.4) (7.0)
Commercial paper    
Debt Securities, Available-for-sale [Line Items]    
Less Than 12 Months, Fair Value   32.1
Less Than 12 Months, Unrealized Loss   (0.2)
12 Months or Longer, Fair Value   0.0
12 Months or Longer, Unrealized Loss   0.0
Total, Fair Value   32.1
Total, Unrealized Loss   $ (0.2)