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Debt Securities - Gross Unrealized Losses and Fair Value Available-For-Sale Debt securities in Unrealized Loss Position (Details) - USD ($)
$ in Millions
Dec. 31, 2023
Dec. 31, 2022
Commercial paper    
Debt Securities, Available-for-sale [Line Items]    
Less Than 12 Months, Fair Value   $ 32.1
Less Than 12 Months, Unrealized Losses   (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 Losses   (0.2)
Corporate debt securities    
Debt Securities, Available-for-sale [Line Items]    
Less Than 12 Months, Fair Value $ 265.1 199.5
Less Than 12 Months, Unrealized Losses (0.4) (1.9)
12 Months or Longer, Fair Value 183.8 299.1
12 Months or Longer, Unrealized Loss (1.0) (5.6)
Total Fair Value 448.9 498.6
Total Unrealized Losses (1.4) (7.5)
Securities of government-sponsored entities    
Debt Securities, Available-for-sale [Line Items]    
Less Than 12 Months, Fair Value 214.6 107.7
Less Than 12 Months, Unrealized Losses (0.2) (2.5)
12 Months or Longer, Fair Value 16.7 198.4
12 Months or Longer, Unrealized Loss (0.4) (4.5)
Total Fair Value 231.3 306.1
Total Unrealized Losses $ (0.6) $ (7.0)